103 posts in this topic

In my view, diesel engines and fuel efficiency follow a simple formula: Run the engine as close to its torque peak, and you'll realize the best fuel efficiency.


We have a 2005 Dodge Ram 3500 Cummins 5.9L, stone stock engine, no "chip" or exhaust modifications, the truck is just as it came from the factory (purchased new in fall of 2004).  We have a friend with a 2004 model, similar, and we've compared fuel mileage for years now.


I've gotten as much as 25 mpg on an unloaded trip from east of Reno, Nevada to Portland, Oregon.  I've pulled a loaded car hauling trailer (Jeep XJ Cherokee on board) to Moab, Utah and managed 17-18 mpg at interstate speeds.  Even after "lifting" the truck and adding oversized tires plus enough accessories to push the curb weight above 9000 pounds with fuel on board, I've coaxed 22-24 mpg out of the truck on flat interstate runs.


So, how is this possible?  Very simple.  I'm an ex-heavy equipment operator and know diesel engines.  These engines have a quick torque rise, more so the Cummins among the light truck applications.  This means that torque comes on quickly, peaks as horsepower builds, and the overall rpm range of the engine is way less than a gasoline engine.


Note: My lesson for all this was running heavy equipment "in the day", primarily with 1693 Cat engines: At 893 cubic inches, these inline six monsters would reach 1090 lb/ft peak torque by 1000 rpm—that's just off-idle!


Our friend seldom achieves more than 18 mpg from his Dodge Ram 2500.  He also has the NV5600 six-speed manual transmission, and I have the "inefficient" 48RE four-speed automatic.  What's wrong with this picture?  The engine operating rpm and our driving technique differences.


A few years back, I asked my friend what rpm he uses for shift points.  His rely was 2500 rpm.  The 5.9L Cummins H.O. inline six peaks its torque at 1600 rpm and redlines at 3400 rpm.  In my experience, optimal fuel efficiency on the highway with this engine has been in the 1600-1900 rpm range, the best mileage achieved around 1600 rpm when not under load. 


Overall, for fuel mileage, the shifts points for this engine should be 1400-1600 when unloaded, 1600 if possible when loaded.  There are times, of course, when the 1600-1900 rpm range is necessary to keep a load moving, and even higher rpm may be necessary for acceleration and climbing grades.


When I modified the '05 truck with the lift kit and aftermarket accessories, adding a good deal of weight in the process, the original axle gear (3.73 with OEM tires) was no longer viable.  While I believed the "overdriving effect" of oversized tires might benefit mileage, the added load and taller gearing effect actually decreased mileage dramatically—especially trailer pulling.


In selecting axle gear sets to compensate, the new 35" diameter tires required 4.10:1 gears for a direct speedometer correction.  I considered the new, unladen weight of the truck and our plans to pull trailers.  My choice was to go even lower (numerically higher) on the gearing. 


The AAM 11.5" and 9.25" axles do not offer a ratio between 4.10 and 4.56:1, so I went with 4.56:1.  This raises the engine rpm at a given speed when compared to the OEM gearing with the original tire diameter.  For trailering and the new vehicle weight, I thought the trade-off worthwhile.


Note: Cummins actually recommends 2100-2400 rpm for peak efficiency in commercial use of the ISB diesel engine.  They would like to see 2100 rpm at 65 mph and no operating below 1900 rpm under load at highway cruise speeds.


In stock form, 1900 rpm netted approximately 69 mph.  Switching to 4.56:1 gearing, 2000 rpm (with overdrive at 0.69:1) nets close to 65 mph.  True to my expectations, my peak fuel efficiency is now at 65 mph or lower, a calculated change. 


If I hold the truck to 55-65 mph, the unloaded peak mileage is 23-24 mpg.  For trailer towing purposes, California caps at 55, Nevada allows for 75 at best, with 65 being plenty of speed for trailer pulling—if you care about mileage...This means watching rpm during upshifts as well. 


I'm very pleased with this truck's fuel efficiency.  Perhaps a chip could improve this further, although my belief is that driving technique holds far more sway over fuel efficiency than any other factor.  If you have a 5.9L Cummins and would like to experience better fuel efficiency, watch your tachometer. 


If I creep over 1950 rpm, the price will be a linear increase in fuel consumption.  By 2100-2200 rpm, fuel efficiency, reflecting load as well, begins to drop like a rock...



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    This is a topic I need to get in on. I still have stock gearing (3.73 I believe) on my 06 mega cab with 37" tires...Now I get a warm fuzzy when I look at the overhead read out and it says 19, but if you stop, hit the calculator up, and do the math at the pump...well that overhead console is a liar...lol! It's more like 12-15 on a good day.


     Now I understand the gearing is a major factor in my situation so I have late summer plans for a set of 4.88's front and rear (thanks to your articles and videos on the AAM 11.5 and 9.25).  I have researched a lot, and I believe the 4.88 to be a viable match to the Toyo 37"??


    I must admit my build wasn't based on the ultimate mpg, or else I wouldn't be running 37's and a 6" lift, but I don't see why I still can't get the most out of this set up. If there is one thing I have learned about mpg's is speed/rpm. Like you stated above, these things can really make the difference even on a heavy 9000#+ truck.  

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Thanks for catching this post...It's among my favorite subjects, as you might have guessed...


Once the axle gearing is correct, the other factors that drop fuel mileage on your '06 Ram 3500 Cummins would be 1) the increased vehicle height (kiss off aerodynamics of any kind!) and 2) the vehicle's weight over stock.  I wound up in a similar situation with a 4" lift, 35" tires and a carload of "cool" accessories!  Not sure of your accessories, I added approximately 1,350 pounds to my over-the-road, "unloaded" weight...kind of like perpetually pulling a well equipped tent trailer! 

post-1-0-02713100-1372477772_thumb.jpg post-1-0-67307300-1372477767_thumb.jpg post-1-0-06461600-1372477770_thumb.jpg

Hey, we all like the "look" and utility of a lifted and accessorized Ram 3500 4x4! Here, the truck we purchased new in October 2004 is undergoing a metamorphosis in 2011, getting ready for show time at the BFGoodrich Tires booth, Off-Road Expo at Pomona, CA! Let's see now, the lift, wheels and 35" tires, we'll add a utility fuel tank that takes us to Moab, Utah and back from the Reno, Nevada area...and that M12000 Warn winch will be a dandy when needed! Oops, there went the 25 mpg. Time for a 4.56:1 axle gear change out!

(Can't see the photos? Join our free forums and get the full benefits of membership!)


Most have no idea how quickly the upgrades and accessory weight add up: Try oversized American Eagle wheels and BFG tires for at least 150# over stock including the spare; a Mopar lift kit after swapping out OEM parts for an added 50 pounds; a Warn M12000 winch for 140# (bare winch wound with wire rope); front and rear HD bumpers for an extra 300#; a Transfer Flow cross bed fuel tank with additional fuel on board: 75 gallons @ 7.1 lb/gallon for Low Sulphur diesel = 532.5 pounds when full plus the aluminized steel tank's weight!  Oh, and I do like the three Bestop Treksteps for 60 pounds plus.


I'll comment on your gearing projections, just did the math...If your tire's revolutions per mile are around 560 (Toyo rating for several popular 37" diameter tires, confirm your exact revs per mile), then here are your engine speeds at practical road speeds in overdrive (0.69:1):


4.88 gears @ 70 mph = 2200 engine rpm

4.88 gears @ 65 mph = 2043 engine rpm

4.88 gears @ 55 mph = 1728 engine rpm


4.56 gears @ 70 mph = 2056 engine rpm 

4.56 gears @ 65 mph = 1909 engine rpm

4.56 gears @ 55 mph = 1615 engine rpm


According to Cummins, you should use the 4.88:1 gears for a truck under 10000# GVWR and intended for 70 mph cruise.  In my experience, though, if fuel mileage were your sole aim without carrying cargo or trailer pulling, I would suggest the 4.56 gearing.  This would keep you "in the window" for maximum fuel economy.  However, even a light travel trailer would immediately tip the scale toward taxing the engine, which could impact both fuel efficiency and engine life—plus overload the transmission (clutch if manual) and driveline.


Actually, with your 37" tires, the 4.56:1 ratio would be much like your 3.73:1 gears with the Ram 3500's stock tire size.  (That was also before accessory add-ons and the lift, too!)  In overdrive, that off-the-showroom floor truck fell well below Cummins' recommended 2,150 rpm at 65 mph baseline for fuel efficiency and commercial hauling.  I'd again emphasize that 23-25 mpg highway was readily achievable with the stock tires, 3.73 gearing and no load at 65-69 mph (approximately 1800-1950 rpm).


If you pull a trailer very seldom and your add-on accessories weight is modest, fuel efficiency would be good between 55 and 70 mph with 4.56:1 gearing and 37" tires.  If the add-ons are like mine, however, your truck has a load before you stack on cargo!  The 4.56:1 gearing would not be low enough, you'd be better off with the 4.88:1 gears.


Note: This is why I opted for 4.56:1 with the 35" tires, rather than fiddle with 4.10:1, which would have been the direct correction for the bigger tires.  We plan to pull a trailer on occasion—without destroying the powertrain.  Also, as I've shared, between the lift height and added accessories weight, this is not the stock truck any more.


Your decision comes down to load and intended cruising speed.  Considering the height and weight of your Ram 3500 Mega Cab, you'd likely be "happier", performance wise, with 4.88 gears.  When you want fuel efficiency, hold the speed to 65 mph.  If that's too slow and you want to "cruise" at 70-plus mph yet get the best fuel efficiency for that rate of speed, consider 4.56:1 axle gearing.  You can see by the calculations that the engine would be in Cummins' recommended zone of 2100-2400 rpm when cruising at 72 mph (2114 engine rpm) with 4.56:1 gears in overdrive.  With 4.88:1 gears at 72 mph in overdrive, the engine would spin 2263 rpm and eat up fuel.


Cruise speeds above 65 mph will eat fuel, regardless...Moving as much mass as our trucks at speeds above 65 mph requires increasingly more fuel.  Base your choice on what cruise speed you find acceptable on the highway—the faster you go, the more fuel the engine will use...guaranteed!


The acceleration might be marginally better with 4.88:1 gears.  In terms of gear stamina with a given ring gear size (11.5" and 9.25" in our case), the 4.56 gears are actually stronger due to the larger pinion gear head size.  (This is slightly offset by the 4.88:1 additional gear reduction, which helps reduce load a bit.)  Given our Ram 3500 ring gear sizes, the stamina distinction is not as severe—nothing like sticking 4.88:1 gears in a Dana 35 Jeep rear axle with a 7.625" diameter ring gear!


We can kick this around more, Megatron.  Cummins recommends spinning the engine for "efficiency" and, at least commercially, does not want to "lug" the engine below 1900 rpm at highway cruising speeds.  Note that a truck under 10000# GVWR with an H.O. 5.9L Cummins ISB engine is less susceptible to lugging than a Cummins ISB engine in a medium-duty truck.


If you're running an aftermarket performance module or "chip", or have done any other tuning or engine modifications, we need to discuss those variables, too...That could change the rpm scale for maximum performance and fuel efficiency, in turn shifting the rpm band for the gearing.



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Didn't see this reply. Kind of covers my questions on oversized tires lol.

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I must still like writing...Spent time doing a fresh reply at your new questions—guess they're well covered now!



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I checked my MPH vs. RPM on the way into work today and my truck says @ 65mph I am turning 1750-1800 rpms in overdrive... so ya that's off per the requirements lol. I assume the speedo to be pretty close to correct given a few neighborhood speed trap signs ha-ha. Well I now know I have some work to do in this area.


   Ok, I do have a good question.  Why does my MPG go up the warmer it gets outside? Does the cold weather rob that much energy from the engine just keeping it warm? Is it possible the winter mix fuels at the pump are less efficient? 5 months ago when it was 20-40 degrees out I was getting 12-13 on the same trip every day. Now that it is 80-90 degrees + out I'm back up to 16-18. I have made no changes to driving style, route or vehicle modifications. Now, I know a little can be lost to letting the truck warm up but even when it was in the 40-50's I wasn't warming up long enough to call it that and I was still getting less mpg. Am I crazy or is there science and math behind it?


 I know diesels don't like to warm up a lot in the colder areas and I know the big trucks even run covers on the front of the trucks to slow down air getting into the radiator or engine compartments.


Any input is appreciated. Thanks.

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Thanks for airing these questions, Megatron!  Given your truck's weight and lift, as we have discussed, the rpm at 65 mph is too low—but only because of the truck's modified curb weight and wind resistance.  I'm not double-speaking, you're at an optimal rpm at 65 mph for extraordinary fuel efficiency with a stone stock '06 Cummins Ram 4x4. 


Proof:  Before I horsed around with 1200-plus pounds of upgrades and a 4-inch lift, in the summer of 2011 I drove to Portland, Oregon from the Reno, Nevada area for the launch of the Jeep JK Wrangler 2012 model (new Jeep Wrangler 3.6L Pentastar V-6 and other changes—see the magazine's JK Wrangler HD video coverage).  I consciously held to 65-69 mph with stock 3.73 gears and tires just under 32" diameter.  The truck had its stock curb weight and height (approximately 7,800# unloaded). 


Result:  I nailed 25 mpg fuel economy from Reno to the mid-Willamette Valley, Oregon!  Best mileage on a long trip ever, including climbs over the Siskiyou Passes and the numerous secondary highway grades between Reno and the I-5 access from Highway 89.  This was not the "Plains" and surely not one-sided in terms of grades.


That said, if I attempted the 1,600-1,750 rpm under the truck's current curb weight and fully fueled load, worse yet when pulling a trailer, I would expect sluggish performance and mileage.  Our Ram 3500 has stone stock tuning to this day, no exhaust upgrades, chips or modifications whatsoever.  I drove this rpm range to achieve this exceptional mileage in stone stock form; however, the truck would suffer with that gearing and current 35" tires. 


Today, cruising at 1,900-2,000 rpm peak, I can coax 21-23 from the truck with the 9,100-plus pound rolling package, unloaded and fully fueled, driving uniform gradient highways...Managed 23 mpg down the I-5 from Sacramento to the 41 Junction en route to Advance Adapters at Paso Robles in January. Held speed to an agonizing 65 mph to achieve this, got passed like I was standing still by the traffic!  Pushed to 68-70 and paid for it immediately:  19-20 mpg with 35" tires, 0.69 overdrive and the 4.56:1 gears.


Last year's trip across northern Nevada's I-80, through Elko to Salt Lake City and on to Moab, I did manage 22-23 mpg at 65 mph on I-80.  Frankly, the Bonneville Salt Flat and Great Salt Lake Basin is tortuous at this speed, although I did have plenty of big truck company in the slow lane.  This is the price for maximum fuel efficiency.  Diesel engines are rpm and load sensitive!  For that matter, gasoline engines fare as bad or worse under load.


We really enjoy the truck and its "look".  (Objectively, folks at trade shows and the Moab Jeep Safari rubberneck to see this Ram 3500 Quad Cab, as I'm sure they do your Mega Cab!)  To a degree, I'm willing to "pay the price". However, my days of 70-75 mph trailer-toting with the XJ Cherokee on board en route to Moab, expecting 17-18 mpg, are over.  I'll still expect 15-16 mpg with a trailer in tow, fully loaded, but only after my change to 4.56:1 gears and "governing" speed to a maximum 65 mph when towing.  Wind is a much larger factor with the 4" lift and 35" tires, an obvious fuel mileage variable...


As a footnote to the trailer toting mileage, I lugged a toy hauler to Johnson Valley for the 2012 King of the Hammers race week coverage.  I still had the OEM 3.73 gearing (like you do) and did respect California's strictly enforced 55 mph speed limit for trucks and trailers, holding speed to a limp along 58-59 mph peak.  I thought the mileage would be great.  Wrong—mileage fell to 12-13 mpg, the worst ever for this truck's trailer pulling and a 7,500# (loaded) trailer.  It's all about the right gearing and keeping that diesel happy under load! 


This raises a point we haven't touched yet.  You're weighing the 4.56 versus 4.88 gearing choice for your 37" tires.  How about this idea: Install a Gear Vendors overdrive behind the transfer case!  The auxiliary overdrive only works in 2WD High range, but you could run 4.88s or, better yet, 5.13 replacement gears in the AAM 11.5 and 9.25 inch axles with the use of the Gear Vendors overdrive when practical—including split shifting on grades with a trailer load.  See the interesting torque gains described at the Gear Vendor's website.  I've linked directly to the Dodge Ram section.


Gear Vendors is a 0.78 ratio or 22% overdrive, which could only be used minimally in tandem with your 0.69 overdrive gear.  Final drive equivalent in overdrive/overdrive would be 5.13 x .69 x .78 = 2.76:1.  This compares to 4.88 x .69 = 3.36:1 [48RE overdrive without Gear Vendors OD]; or 5.13 x .69 [48RE overdrive without Gear Vendors OD] = 3.54.  Your OEM gearing factors currently as: 3.73 x .69 = 2.57.  Of course, we haven't targeted engine rpm and your tire diameter yet...We can play with the gearing equation and see if this makes sense.


Cost aside, the Gear Vendors overdrive is a consideration if you're on the fence here.  ..I've got a one-piece driveline at 140.5" wheelbase, you're longer based with a two-piece driveline.  Plenty of room for a Gear Vendors unit in either case.  For my truck with its current 4.56:1 gearing and 35" tires, I'd have gains like 1) the double overdrives when running empty at cruise or 2) when pulling a hefty load, I could use the Gear Vendors overdrive with the 48RE in 3rd gear (22% versus 31% overdrive, a 9% reduction advantage).  You haven't changed gears yet, so there are more options to consider.


I like your seasonal fuel efficiency question!  Like the intercooler, winter cold air creates denser oxygen content, which increases power.  If your climate is humid in the summer, you may get an additional intake air cooling effect and, therefore, denser oxygen content in the intake stream.  A denser charge means improved combustion and, at least theoretically, better fuel efficiency if you can keep your foot out of the more responsive throttle! 


Another factor to weigh is the OEM or aftermarket computer "chip" tuning that takes advantage of a denser IAT reading by changing injector pulse width.  This could also work another way: When winter intake air is cold and dense, the injectors flow more fuel to create the right air/fuel ratio. The result is more fuel consumption and peak throttle response.


I believe the winter fuel efficiency loss is also due to lubricant viscosity and cold pour, especially in the axles.  (Transfer case lube and ATF are now largely synthetic and wider-range viscosity, typically with a lower weight ceiling.)  It's no small fact that OEMs now use lighter, multi-viscosity lubricants for CAFE ratings. The AAM axles call for 75W-90 weight lube.  My choice of running 75W-140 Mopar gear lube for severe duty use may, in fact, create a loss in fuel efficiency—especially in the winter. 


Don't get carried away with this factor, though, unless we're including engine oil viscosity.  See this technical paper for interesting details on fuel efficiency as it relates to motor oil viscosity: http://www.instituteofmaterials.com/paper/FEIPaper.PDF.  I'm running Mopar 15W-40 year round with use of a winter block heater to avoid startup damage.  Frankly, this could be a source of higher fuel use in winter.  The 5.9L Cummins engine can drop from 195-degrees F to the bottom of the temp gauge during a 30-minute groceries stop! 


Another factor, perhaps significant in our case with oversized tires, is rolling resistance.  In winter, the advertised or usual static cold tire pressures can create drag.  On frigid highways in chilling air, the tires do not reach the predicted temperatures for pressure calculations.  In the summer, there is less rolling resistance as inflation pressures rise due to ambient and road surface temperature increases.  Sadly, tire pressure is fixed (unless you have a Humvee and can air and deflate on the fly!).  You cannot inflate tires to the summer over-the-road pressures for winter use—the tires would be deformed and wear quickly on their tread centers from "over-inflation".  They would also be a terrible hazard with minimal road surface contact!


As for engine operating temperatures, and this is a factor, it does take "forever" for the Cummins engine to warm when ambient temp is sub-zero to 40-degrees F.  This is not a coolant thermostat issue, although 18-wheeler shutter-stats like you hint would be helpful.  The Cummins ISB engine has a huge cooling capacity!  Forget letting the engine idle to warm up, it will take forever to reach operating temperature.


On that note, I'll emphasis that I do use a block heater all winter.  The truck is not a daily driver, but on any day with a planned run, I plug in the block heater the night before.  Under a carport with a Battery Tender hooked up whenever the truck parks, let's say it is a 10-degree F morning:  The engine coolant's startup temperature will be around 120-130 degrees F with the block heater. 


Warmer weather (a whopping 35-degree F, let's say), start-up temperature will be around 140-degrees F.  That noted, I start the engine and always wait for the oil to circulate sufficiently before rolling.  It will sometimes take from Fernley (home base) to four miles West on I-80 before the engine reaches thermostat temperature.  During this entire time, I hold engine speed to no more than 1,500 rpm under light throttle load.  I expect this diesel engine to last for 500,000 miles, enough to bury the initial cost difference over a gasoline engine—and also to save a small fortune through fuel mileage gains over that number of miles.


So, summer does bring better fuel efficiency.  In my case, I've also switched to GDiesel, a natural gas altered fuel.  (See my coverage of GDiesel in an article and HD video interview at the magazine.)  By breaking down long chain molecules to more combustible form, this fuel does deliver mileage gains.  Even better, I do not engulf other motorists in soot when I launch into the throttle during passing. 


Yes, my truck's exhaust, even without the later catalytic converter and exhaust stream additives, is clean, without visible smoke—black or white.  While some clean-up can be produced through ECM programming, that's only to a degree.  I use GDiesel and avoid mixing it with conventional low-sulfur fuel.  (GDiesel is actually the very same low-sulfur base fuel, reprocessed with a patented method.  You can mix the two in a pinch.)  Fortunately, I can make it to Moab from Fernley and back home on one fill of the main tank plus the 75 gallon auxiliary Transfer Flow tank in the bed.


Thanks for the questions, Megatron...Others are welcome to jump into the discussion and share their experiences!



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 Okay, I am onboard with the Gear Venders overdrive setup. While it has an initial cost, it is one that can truly be paid back during its service if better fuel mileage is achieved. I have an Atlas 4SP transfer case with a similar unit on the input side for reduction. I am aware of how they work and mine works fine (on my rock-crawler not my daily driver, although I thought about putting it in my truck.  You have given plenty for me to research, and research I shall do!


  Your points about winter fuel mileage make sense. While I had hoped I could point my finger at one thing and fix that, you have shown it is a multiple of things adding up. I do switch to different oils for winter time, but I was under the impression it was spec'd from the factory this way?


  Your GDiesel is something I have never heard of. I am only a year into the diesel world so I still have much to learn. I plan to check this out as well. I am a fan of science and am amazed how it can effect my wallet on daily driving.


 This may be a dumb question but I thought I would ask, hope it makes sense. At what speed do you think "lack of" aerodynamic efficiency rules out any attempt to be fuel efficient? Now this is not something I can test given my local speed limits, but with my current truck gearing I should be running in the 80mph range (-/+) to be at the desired RPM for engine efficiency. Do you think that I would see a fuel mpg increase at those speeds, hypothetically? At least if I get a speeding ticket I could say I was trying to save some money lol.


  My next real question, that I hope you may have experience with, is water/methanol injection for MPG gains. Have you gone this route yet or maybe anybody that is reading this? The science seems to be there and the posted research seems to back the claims, but I have reservations about the on going cost to replenish the supply vs. the gains. At some point you have to say your still spending money on the 2-3 mpg you saved but is it still cheaper than the price of fuel?? Are there other gains besides a rusty intake plenum that you can take into consideration? 


  As always, thanks for your knowledge and insight on all topics. You are definitely filling my brain with knowledge that is useful and I plan on putting as much of it to use as possible.  

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Well, you're certainly opening my eyes to the value of humor in these posts!  Keep it up, Megatron...


The Gear Vendors is definitely an option to consider before you change axle gears.  Here's some quick math on the Gear Vendors 0.78 ratio overdrive with your 48RE automatic transmission and a change to 5.13:1 axle gearing:  At your favorite highway cruise speed of 75 mph, if you use the Gear Vendors overdrive plus your 48RE's overdrive, with 37" tires (560 revs per mile) and 5.13 axle gearing, your engine speed would be 1932 rpm. 


From my experience with our 3500, this could be close to the optimal cruise rpm for maximum fuel efficiency at this weight, height and speed.  If you needed passing or pulling power, without the need to floor your 48RE for a forced downshift to third gear, you'd simply kick out the Gear Vendors overdrive and be at 2478 rpm.


This is the best of both worlds and does target your 75 mph cruise speed.  If you slow down to even 70 mph, you'd be at 2313 rpm with the 48RE's overdrive and direct gear on the Gear Vendors.  Apply the 0.78 Gear Vendors overdrive, and engine rpm would drop to 1803 rpm.  You get the picture, this looks like some targeted rpm options for optimal fuel efficiency and still above the torque peak rpm in all cases.


By the way, it's very nice of me to help spend your income on items like a Gear Vendors overdrive!  Kidding aside, this could pay big in fuel savings and also offer some incredible torque and pulling options as a split-shifter.  Neither you nor I currently have the advantage of the latest 8-speed automatics—unless you toss the Gear Vendors overdrive into the equation!  This is the wave. 


Again, here is a copy of the Gear Vendors link I provided in my last missive: Gear Vendors overdrive when practical.  Gear Vendors is in the gear business, and they build an impressive case for torque gains through nothing more than gear ratio changes and split shifting.  The OEMs have obviously gotten the message with the new wave of 6- to 8-speed automatic transmissions—which will make manual transmissions in light- and medium-duty trucks a thing of the past.  Before taking the plunge, consider your GVCW (gross vehicle combination weight) with the trailer in tow and the stamina you can expect from a Gear Vendors overdrive.


As for winter oil changing and specs, consider the wide range of climates and engine operating temperatures that OEMs address.  Oil recommendations must be generalized and take into account the temperature range from startup to warmed operating temperature under load.  Dicey choices!  0- or 5-wt. motor oil is great for cold pour in Alaska—but not for the very same engine running at 195-degrees F thermostat temperature once it warms up.  Minus 50 F to 195 F is a wide temperature range—from cold start to fully warmed up—a typical Fairbanks, Alaska day in the dead of winter!


Regarding the 80 mph method of getting the engine to rpm, save the speeding ticket.  (Try running to 80 mph on some secluded road and let me know whether the engine likes it...Watch your rear view mirrors for flashing red and blue lights.)   Note that when you increase load like with a trailer in tow, you may need horsepower in addition to torque.  The Cummins 5.9L ISB engine is under no stress at stock 7,800 pound curb weight and OEM height;  loping along at 1,600-1,900 rpm seemed just fine under that scenario, with road speed peaking around 69 mph.


To push our "billboards" down the road at 80 mph creates high rolling and wind resistance.  You did the math perfectly for your 37" tires, 3.73 stock gears and 80 mph in 0.69 overdrive:  1,922 rpm.  The question is whether the engine would be happy at that rpm under heavier loads. 


I used to periodically run our truck to 100 mph in a sprint up a particularly secluded highway in the desert.  The grade was over 6%, and I would start at the bottom around 65 mph and accelerate to 100 before the crest of the relatively short grade.  With the truck's current weight, the lift, tire drag and even the right axle gearing, the engine would perform this feat under load now—even with nothing in the bed but the auxiliary fuel tank.  Times change—we still like the look and utility, though, right? 


Now we're talking about the constraints that drive Cummins to declare that our engines should never run below 1,900 rpm under load, and they want to see us running 2,100-2,400 rpm with "commercial" loads.  Driving your lifted truck 80 mph is a pretty good load, perhaps comparable to "commercial use" standards. 


Here's engine performance data on the Cummins ISB 24-valve 5.9L inline six: http://www.cumminsdieselspecs.com/24v.html.  Our engines peak their horsepower around 2,900 rpm.  Most ISB commercial applications have 3,200 rpm governors.  Chrysler and Cummins added a couple hundred rpm for those Ram owners who think they're driving a gasoline powered vehicle, even though the engine's power has fallen off considerably and shifting to the "next gear up" would be a better idea. 


The horsepower rpm is worth considering.  While peak fuel efficiency at a light load is 1,600 rpm, without getting into physics, we can assume that if we continue adding a load to the engine, we will need more horsepower.  An H.O. engine peaks at 325 horsepower, quite impressive and requiring 17.2:1 compression.  So, you're right in suspecting that as the load rises, fuel efficiency suffers. 


Of course it takes more power to move over four and a half tons of mass at 80 mph.  Then there is the aerodynamic (lack of?) drag coefficient we know is lurking.  In terms of drag coefficient, all I could drum up on line was for a stone stock model year 2000 Ram 3500 4WD Quad Cab:  0.48 rating.  If you'd like to see how this stacks up against other vehicles, ranging from sleek race car models to popular econo-boxes designed for peak fuel efficiency, or even some SUVS (much like our trucks), check out this well-done Wiki entry: http://en.wikipedia.org/wiki/Automobile_drag_coefficient.


The cool thing about the diesel engines in our Ram trucks is that the torque stays well up there from 1,600 rpm to at least 2,700 rpm!  This means that unlike a gasoline engine, the torque and horsepower don't go separate ways with advancing engine speed.  Diesel technology does require a re-think on the part of gasoline "performance" enthusiasts who see horsepower figures as an end all:  Unlike diesel torque rise, high horsepower figures are only attainable at higher rpm.


This even applies to the popular Jeep 4.2L/4.0L hybrid "stroker motor".  Begin with a reliable inline six.  Build a "hot" stroker for 300-plus horsepower, and discover that to achieve this kind of power, you'll be pushing the torque and horsepower peak rpm way up the scale.  Here are figures for an exotic Jeep 5.0L stroker gasoline engine built from a 4.0L inline six block with a custom stroker crankshaft and 11.5:1 compression:  344 horsepower @ 5300 rpm and 384 lb/ft torque @ 4000 rpm.  Great performer for sand drags—lousy power curve for the Rubicon Trail!


The analogy for a Cummins diesel is that maximum high performance/horsepower builds (those short lifespan engines that Gale Banks talks about) mean spinning the engine to oblivion to achieve peak power.  Gone is the famous torque rise that distinguishes a long stroke commercial diesel engine—or a stock 4.2L Jeep inline six for that matter—from the typical gasoline engine...


Regarding the principles of water/methanol injection and its history, here's a nice ditty: http://en.wikipedia.org/wiki/Water_injection_(engines).  A water/methanol injection kit like the one offered by AEM reminds me of one of my earliest freelance magazine pieces.  We had a '73 Chevy K10 4x4 SWB pickup with a 350 V-8.  I installed a "solution"  injection kit that was popular in the '70s.  That somewhat sophisticated system came with a thick carburetor base gasket that had small orifice tubes running into the throttle bores.


A needle bleed valve at the glass solution jar metered the water/alcohol mix.  A slight bubbling indicated proper adjustment and flow.  Pretty slick, actually, the device did what water and alcohol/methanol injection will:  On a gasoline engine, it cooled and condensed the incoming fuel/air charge, reduced ping or detonation, allowed for more spark advance and better power, and permitted use of higher compression ratios.  Such a device allows fuel timing and compression changes on a diesel as well. 


Not often mentioned, during the combustion process, the water/alcohol or methanol solution will remove carbon from the combustion chambers, valves and piston crowns.  Upon the 350 V-8 teardown, the cylinder heads and upper engine looked virtually "new".  In this modern era of electronic fuel and spark management, carbon buildup is a virtual non-issue.  In the era of the Quadrajet carburetor, however, fuel enrichment and "venturi effect" made carbon deposits at the upper cylinder and combustion chamber areas a chronic engine problem. 


Fuel efficiency improved dramatically, the 350 V-8, gasoline powered truck delivered as much as 18 mpg with 3.08 gears, a 1:1 fourth gear ratio in the SM465 four-speed and 33" diameter tires.  Performance improved noticeably from added spark advance—without signs of ping/detonation.


For your diesel, this injection would act like an additional intercooler, keeping exhaust gas temps lower for less stress when trailering or hauling a load.  The upper engine would be cleaner, possibly extending engine life.  As with GDiesel fuel, crankcase contamination from incomplete combustion would be decreased.  Combustion would be more thorough.


Is this enough to outweigh the cost of methanol/water injection?  Can't say for sure, but this is why I buy GDiesel locally for 10-12 cents a gallon more than other low-sulfur diesel pump fuels.  I'm "banking" on the engine yielding a cleaner tailpipe, cleaner crankcase (lab tested the oil at Pape Cat, it's holding up much longer and shows fewer contaminants at change intervals) plus extending engine life.  If water/methanol were used for this purpose, I might buy it... 



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Moses, Megatron,


First off well done. I have to say after hunting for quite some time that this thread holds some of the most pertinent and well put information that I have been able to find. I have an '02 3500 ram dually that I tow with delivering travel trailers and 5th wheels across the country. I put a lot of miles on and half of that time under load of generally 8 to 15,000 pounds of tow weight. I have the automatic transmission and 4.10 rear end. I am no mechanic and a lot of what I read is greek to me so I'm hoping to get some advice from those that I believe are in the know.


I drive dead heading with my RPM set at 2000 splitting the line which my gps reads out at 65, and towing as well with my O/D on unless i'm pulling mountains and then I turn it off only when the hill starts and I know its enough to drop my gear and turn it back on when I crest. I'm making 19-20 miles per gallon (on my overhead display) dead heading and 10 (average) when hauling despite the weight. This is with the truck fully stock (25 gal, reserve tank in the bed, no tailgate). This weekend I went out and upgraded the exhaust to 4" from the turbo back with an aero muffler and picked up a K&N filter (despite some folks dislike of them, only ever had good luck myself). I am hoping to see some better fuel efficiency from the move but both were out of necessity (exhaust rusted out and I got tired of paying for a paper filter every-time).


I am only looking for efficiency as it affects my bottom line. I have no interest in increased power or noise, in fact I almost went stock with the exhaust system again for that very reason, I have to be able to live with the truck spending 11 hours in the seat a day and sleeping in it more than occasionally. This seems long winded, so I'll get to it, I'm looking for advice on fuel efficiency, i.e. -maintenance practices, upgrades, and most importantly driving techniques, I have no idea what my optimal peak RPM is? and with that being said, longevity of my truck is foremost. I would like to see her join the million mile club someday. Let me know what ya think...

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Welcome to the forums, Demerchant!  I'm pleased to see your Ram delivering such good service and mileage.  The weight involved is phenomenal, a real testimonial and chapter for the "Cummins story".
As for maximum fuel efficiency and longevity of the engine, you're on the right track.  Your K&N choice did come with the general concerns about gauze filters and adequate filtration.  You drive highway, however, so dirt is not as much of an issue.  The exhaust upgrade should be a plus, certainly no down side.  If you continue to drive for economy and not to "see what the new exhaust and K&N air filter can do for performance" (well, maybe a little bit, but at the sacrifice of mileage when you do!), then these add-ons will be help with engine breathing and exhaust flow.
Your 19-20 at 65 mph dead heading in overdrive sounds reasonable.  The dually weighs a bit more than our '05 Ram 3500 SWR, and at 65 mph with 3.73 gearing, no load (also before the auxiliary fuel tank, massive winch/bumper, etc.) and stock tires, I could squeeze 23-plus mpg highway most of the time.  I did accomplish a one-time high of 25 mpg on the highway, achieved by keeping the engine between 1600-1900 rpm faithfully over a 480 mile trip.
Again, as Megatron and I have discussed, engine rpm is the key to fuel efficiency with any engine, and in particular the Cummins diesel.  Our '05 HO reached peak torque (stone stock) at 1600 rpm, and driving between 1600-1900 was the sweet spot.  If I pushed beyond the 1900 mark, fuel efficiency dropped considerably.


When I added 1200 pounds of accessories and auxiliary fuel, the new dynamic, plus oversized diameter tires, required 4.56 gears.  I'm now in the same league as your 4.10 gearing with your stock tire diameter.  69 mph is now 2100 rpm.  65 mph is a hair below 2000 rpm (1980-1993 rpm depending upon accuracy of the current tire diameter).  65 mph will return a consistent 21 mpg on highway cruise without a trailer, even at full fuel load (110 gallons) and the truck's weight hovering around 9,000 pounds.  69 mph means 20 mpg, often dipping to 19.8 mph with hilly or mountainous highway terrain.
You're already driving for economy at 65 mph peak.  If your tires are stock at around 31.9" diameter, with 4.10 gearing and 65 mph, I have the calculation at 1937 rpm in overdrive.  (Does that sound right?)  This is near the magic zone of 1600-1900 rpm for the later 2005 HO 5.9L ISB Cummins.


As a comparison, the 2002 Cummins HO and standard engines reach stock peak torque by 1400 rpm.  (Please confirm whether you have the 250 hp "standard" output engine.  As I understand, the 2002 HO 305 hp engine was available only with a manual transmission, not your 47RE automatic.)  Chrysler kept the lid on peak torque output with the 47RE transmission then turned the torque wick up on the 48RE with advertised horsepower at 325 and 610 lb-ft torque.  (This is wonderful as long as the transmission can stand it!  If we had not purchased our truck for multiple drivers, I would have opted for the 6-speed manual transmission.)  Your 2002 Ram 3500's peak torque should be 460 lb-ft. (The HO with manual transmission rated 555 lb-ft in 2002.)  Torque peak for both the 2002 standard and HO engines was at 1400 rpm.  Our 2005 peaked its torque at 1600 rpm in stock tuning.


An interesting footnote is how flexible the power can be on these Cummins diesel ISB engines.  The 2002 HO's 555 lb-ft torque was quite a boost over the standard engine.  Most of this difference is tuning or tune programming for these common rail, electronically fuel injected engines.  When I decided to reprogram the '05 engine with a Hypertech Max Energy tune, the torque peak went up, achieved at 2100 rpm instead of 1600.  This was a boon, as the engine now reaches peak torque at 68-69 mph and can deliver 20-plus mph on the highway even at that rpm and road speed.


Torque peak is the key to fuel efficiency on these engines. Theoretically at least, your engine and fuel economy should be best between 1400-1800 rpm.  A test for improved fuel efficiency might be to drop your cruise down to 1800 rpm (59 mph or so) and check the mileage.  It should go up.  Even slower, like 55 mph (1700 rpm), should deliver better mileage yet. 


This raises a practical consideration, though.  Try confirming my suggestions for improving mileage on a flat stretch of road when you have the time and patience!  Since there is also the sensible need to drive your truck at a reasonable pace, you may find that saving fuel is less important than getting to your destination in a reasonable timeframe.


Note: According to Cummins' commercial site, the ISB commercial version of this engine should be run between 2100-2400 rpm for maximum efficiency (presumably including mileage).  That's with commercial engine tuning/programming and intended loads to 50,000 GVCW.  In my experience, that rpm would drop the mileage to the 17-18 mph range unloaded.  With commercial tuning, however, it might help the heavy hauling tow mileage.


There are few design differences between our engines.  Hypertech taught me that tuning is everything: power, fuel efficiency, rpm for peak torque and peak horsepower.  (Click here to see my full explanation of the re-programming approach.)  A Max Energy program for your engine could make a difference in two ways:  1) an overall increase in performance and 2) bumping the peak torque rpm up, which would make your 2000 rpm cruise speed more viable for fuel efficiency.  Understand that with any diesel engine, shoving more fuel and air into the engine increases power and often efficiency.  I'm not clear about the number of miles or wear on your engine, and if you consider a re-program, do not alter your driving habits.  Keep a lid on the engine exhaust temperature (EGT) under load!


This last point is crucial.  Hypertech assured me that its programming is "tow friendly".  Your scenario would be the ultimate test, considering the loads you pull!  I would not do a program change without installing an EGT meter.  This should read exhaust temperature at the pre-turbo point if possible to assure an accurate read of actual exhaust temperatures.  The turbo can cool exhaust readings considerably, so "post-turbo" probe installations can often be inaccurate. 


Warning: Use extreme care with drilling into the manifold pre-turbo: Metal filings and residue can destroy the turbo!  Some use grease on the drill bits and tap.  (Some argue this is not a good idea.)  There are other techniques we can discuss if you install an EGT kit yourself.


This sums up as two options for better fuel efficiency:  1) slow down the engine speed (either lower the vehicle's road speed or install a Gear Vendors overdrive for dead heading) and/or 2) do a tuning program change like the Hypertech with the proviso that you monitor your EGT as a safeguard.  We're all on board with making the "million mile club" with these trucks...If you need an incentive, just price a new Ram/Cummins 3500 dually!


As an anecdote from third parties, I was parked at a local fueling station alongside two Dodge Cummins trucks (an '04 and '05).  The owners, a father and son, commented that our 5.9L models get much better fuel efficiency than the later 6.7L models.  Better yet, and consistent with your plans, they had an '02 that ran to 600K miles before selling it, and the father insisted that the truck is still out there doing fine.  It had a "chip", the type and brand not discussed, and according to these fellows, the mileage and performance were impressive.  All three of their trucks were automatic transmission models, there was no discussion about transmission rebuilding.


Let's keep this discussion going, Demerchant.  The cost of fuel won't be going down any time soon!



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 I'm a camper jockey the same as Demerchant, and I could use some help.


Truck specs are as follows:  02 Dodge 2500, 8' box, quad cab, 4wd, 3.54 gears, 250k miles.   Completely stock, with the addition of a low fuel pressure light, a ScanGauge II, and a Smarty S03 programmer set to level 3 (towing).  I also have a FASS DDRP lift pump, and a bed mounted 50 gal. fuel tank.   Empty weight (without the bed tank), loaded with my gear, was about 6600 #.


The best fuel loaded economy I've managed was 14.3, running from IN to NC with a strong tail wind.   Empty?  Maybe 22 mpg.    Averages are usually more like 10.5 loaded and 18-20 empty.   Round trip mileage usually comes out to 14. 


Loaded speeds are kept to 60-62 usually, downshifting to 3rd when climbing hills.   I set the cruise at 62 when I'm running home empty.  This keeps my rpm around 1600 in 4th.   I lower speed in 3rd, according to the mountain I'm trying to climb, usually 50-55



I'm struggling to get good mileage out of this rig.   I started in this business running a worn-out Chevy 2500 2wd, with a 6.5TD.   I bought this Dodge a few months ago, hoping for more mileage and easier towing.   The Chevy would deliver 22-24 empty and 11.2 loaded, consistently, driven the exact same way.  I dropped a pretty penny on this truck, because it had a reasonably straight body, new factory transmission (with single disc billet TC), and a completely rebuilt fuel system (new stock injectors, VP44, and DDRP lift pump about 2 years ago.)   I'm planning on adding a FASS Titanium 150 LP when I get my taxes back for longevity and reliability. 


Really hoping to have a million mile motor on my hands, but I've got to do something about fuel economy.    Fuel prices are bad enough here in IN, but I spend most of my time in The Great White North, where fuel prices *start* at $5.25/gal.   I had this crazy idea about lowering the truck, but can't figure out how to get around the issue of oil pan-front axle clearance.


Thanks in advance



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Hi, Paul, welcome to the forum posts!  To begin, the weight of your truck as equipped may be higher than 6600# when "empty".  Is this over-the-scale weight or speculative?  Our '05 Ram 3500 4WD Quad Cab SRW with short box was somewhere around 7,800# advertised curb weight.  I've searched around the internet, and owners and information sites are throwing out figures of 7,100 to 8,100 pounds for stone stock vehicles, not being clear about completely empty or with passengers, fuel and so forth.  I think we need to weigh our trucks, calculations for my truck are not actual scale weight.


So, it would be wise to start with an accurate weight figure, and if you get one, please share.  (We'll start using actual weights rather than advertised or speculative weights.)  Also, I'd like to know the loaded truck weight whether it's in-bed campers or travel trailers in tow.


I understand your frustration and am looking at your gearing.  3.54 is tall.  Our truck had 3.73 gears when new, and without all of the add-on weight of accessories (lift kit, tires/wheels, Warn massive bumper and winch, Transfer Flow fuel tank for 75 gallons more fuel and so forth), we consistently got 22-24 mpg at 65-69 mph.  At the conservative speeds you describe, running empty, I would achieve 24 mpg or a bare minimum of 23 mpg.


If your tire size is the same as our '05 was stock, you should get better mileage than you're finding.  Before commenting further about your tall gearing, I would like to know the tire diameter/size.  3.73 gearing seemed the magic number for our stock '05 tires at 65 mph, I caught an all-time best of 25 mpg with this arrangement on an empty run from the Reno Area to Portland, Oregon, holding speed to the 65 mph range.  This was one time.


Gearing is crucial.  With the added equipment, weight and oversized tires, which essentially gave me gearing more like yours would be with stock tires, the fuel efficiency actually fell off.  I thought optimistically that the overdriving effect of the 34.6" (advertised 35") tires would possibly improve mileage, and it didn't.  Fuel mileage dropped dramatically, especially when trailer pulling.


So this tells us something about the sensitivity of gearing.  For the oversized tires, 4.10:1 gears would have restored the rpm and load to stock, but I wanted more pulling ability for the added weight and anticipated trailer pulling.  There were no AAM gear sets available at the time between 4.10 and 4.56:1, and after pondering quite a bit, I opted for 4.56 gears.  (You can see my install article and video at the magazine site, use the term 'AAM' in the Search Box.) 


I knew it would be necessary to hold speed to 65 mph for fuel efficiency, 69 is tops now, and my mileage varies accordingly:  On flat ground, 65 mph is good for a consistent 21 mpg average with full tanks of fuel, passengers (2), luggage, empty bed and no trailer in tow.  I've "squeezed" 23 mpg on flat I-80 runs with a tailwind at 65 mph.  Step up to 69 and pay the price:  Mileage is more like 20 mpg and drops to 19 in hilly country.  Emphasis is that the truck could weigh over 9000 pounds going down the road "empty" as described.  (Again, I need to get valid scale weight here!)


3.54 gearing is quite tall, however, your engine has a different torque peak speed (lower) than our '05 HO.  Not sure what the dyne figures show for the "Smarty" reprogramming, if you have peak torque ratings from their testing, we can take that into account.


I'm not comfortable spending others' money and time on gear changes unless there would be obvious gains.  Please clarify your loaded weight (estimate at least), whether you are hauling in-bed campers or travel trailers, the tire size and the torque rating with the Smarty programming.  Also, does the 'ScanGauge II' include a pyrometer?  We'll discuss this further...



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I'll do my best here.


The ScanGauge provides readouts from the truck's OBDII system.  (https://www.scangauge.com/products/scangaugeii/ for reference).   I use it to keep tabs on IAT and coolant temps, as well as a "guessimate" of mpg.   (All of my mileage figures are hand calculated, from the same station, same pump, unloaded every time.   I fill up at the beginning of every trip, and log fuel purchases and mileage until the next time I can fill up at my favorite station.)


The 6600# weight was from the scale at my local gravel pit.   ~30 gals of fuel, driver + passenger, my toolbox/chains/straps/etc onboard, but not my 50 gal aux tank.   My scale ticket from Alberta on 4/1/14 says 1960 k (4321#) front axle, and 1350 k (2976#) rear axle.   Truck, with me, my gear, and about 50 gal. of fuel onboard, no trailer.   That is a certified Canadian scale, so it's probably the most accurate.   Total weight of 7297 pounds.


Tires are Firestone TransForce A/T, 265/75R16, on stock steel rims.   About 50% tread, run at 65 PSI. 


I run bumper pull travel trailers or horse trailers.   Usually no 5th wheels right now, no in-bed campers at all.  Curb weight and sizes will vary, from 2000# up to 10,000#, 20' to 37'


I just bought this truck a couple months ago, haven't had time to get it completely outfitted yet.   I've got a set of gauges, boost/pyro/temp.   The only one installed right now is a trans temp gauge, mounted in the pan.  The others should be going in this week, as I've got some time off.   I will also pull a diff cover (probably the front) to verify the gear ratio of 3.54.


I'm also going to try and make a couple of pulls on the dyno at a local automotive college.   I want to make a pull with stock ECM programming, as well as a couple with the Smarty on different settings.  


I don't have much experience with these trucks, having mainly run bigger, IH and Cat motors in my past, where fuel efficiency wasn't nearly as much of a concern.  I know there are aerodynamics issues with this big of a truck, things that I will be addressing.   My only other comparison is word of mouth from other drivers.   Just met another yesterday, with an identical rig, except 2wd.   He claimed to get 14 loaded, 25 empty, but didn't have any numbers to back it up. YMMV, as they say

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Paul...I'd cleave toward the Canadian readout of 7,297 pounds, still less than I expected.  The tires are stock size and like my '05 before the change.  You're running a lot of pressure when empty, so they have more than enough air.  (Are you okay with running this kind of pressure empty, or do you drop down to door sticker settings?)  Sounds like you're pulling good size trailers but not pushing "billboards" down the road as far as wind resistance goes; the tall 5th wheels are the billboards.


Would like to hear if the gear ratios on the axles are other than 3.54:1.  Glad you're into the pyrometer.  When running aftermarket programming and these kinds of loads, you do need the EGT monitoring to keep a lid on the engine stresses.  EGT is the best means for monitoring the load on a diesel.  I need to get a pyrometer for the '05.


Really would like the dyne results from the community college testing, how fortunate that you can use this dynamometer.  Stock would make sense as a baseline.  This is exciting, Paul!


The 25 mpg is not out of reason for a 2WD empty with the correct gearing.  If he drives like we do, for mileage, 25 is conceivable.  I place great stock in the right gearing, the Cummins ISB engines are hypersensitive to rpm when it comes to fuel efficiency.  Under commercial loads (to 50K GVCW, apparently a medium duty truck application, not my one-ton with a 48RE transmission, for sure!), Cummins would like to see the ISB engine running in the 2100-2400 rpm range.  I wouldn't expect more than 17-18 mpg unloaded and 14 mpg pulling a 7,500# trailer at that rpm.  When I push to 2100 rpm empty, which is actually my presumed torque peak with the Hypertech Max Energy programming, I can expect 19-20 mpg, depending upon terrain and wind.


This last factor, wind, cannot be underestimated.  I raised the truck 4" with a chassis lift and added oversized tires, which is clearly a red flag for fuel efficiency.  These trucks are hardly aerodynamic to begin, and I asked for it with this move.  Since all modifications to my truck were made simultaneously, there is no way to separate the impact of the lift and broader frontal area, i.e. increased wind resistance, from the rest of the changes.


For that reason, I work with the known factors:  1) driving for efficiency is something I can control, 2) gearing is intended for a top speed of 69-70 mph if fuel efficiency is primary, 3) gearing (4.56:1 with 34.6"/35" tires) was picked to maintain the lifespan of the engine first, as a fuel mileage consideration second, and 4) I take full responsibility for the impact of the lift, tires, gearing and all the accessories added to the truck!  This last point is an important qualifier.  I can't complain about fuel mileage after doing all of these modifications intentionally.  I'm in damage control mode at this point.


Let's keep working on your mileage.  I appreciate your approach and the information gathering that you're willing to do.  We can work with these facts and find a sensible strategy.





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OK, after a bit, I'm back.


I took my truck to the local dyno, and got some mixed results.   I was having troubles with the transmission wanting to shift, and it was giving me some mixed readings on torque and horsepower numbers.   Basically, every time the transmission would shift, it would peak the torque curve.   This seemed to happen about 2200 rpm or higher, under WOT.  


The closest thing I can guess, I did have a few runs that showed a peak torque between 1800 and 2000 RPM.   I'm working on a few other sources of info, to see if I can get any more accurate information.   May also be able to make another dyno run in a few weeks.


I had a issue with fuel pressure up in North Dakota last week, ended up installing a FASS 150 Titanium lift pump.   Truck is running great now, but for some reason my fuel gauge doesn't work now.  It seems sometimes that if it's not one problem, it's another, right?


On my last run back from Colorado, I seemed to get the same ~20 mpg whether running at 70 mph (1900 rpm), or running 65 (~1750 rpm).   I know that going from fillup to fillup across states isn't the most accurate method of measuring, but it's what I have to work with.


While regearing axles isn't really in the cards financially, I will be needing to replace my tires in the next few months.   This should allow me to go smaller to adjust for the gear difference, if necessary.

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Umm...For the stock ECM, the 1800-2000 rpm torque peak is higher than expected.  This would be closer to an aftermarket tune or programming.  Dyne tests by Hypertech on the program now in the '05 Dodge Ram 3500 show peak torque at 2,100 rpm; Chrysler factory programming was rated for peak torque at 1,600 rpm.  My fuel efficiency sweet spot, again, is 1980-2000 rpm maximum, which would have the torque at or near peak.  This is a reasonable road speed of 65 mph.


It is not surprising that you would get 20 mpg at both 70 and 65.  The rpm range may reflect peak torque versus lower engine speed.  Typically, mileage does improve with less speed, but if torque falls off too much that is not necessary the case.


In considering your quest for maximum fuel efficiency, aside from correct gearing and tire diameter, another aid is a conversion from unit hubs and the front axle constantly spinning to freewheeling hubs.  A full-floating hub conversion (resembling the "real" 4x4s of a bygone era) allows use of freewheeling hubs to disconnect the drag of the inner axle shafts, differential gears and front driveline when in 2WD mode at the transfer case.  Manual freewheeling hubs make this a true disconnect, and the full-floating spindles, wheel hubs and inner/outer wheel bearings offer superior load carrying capacity and lifespan.  This setup also allows for periodic wheel bearing service/lubrication like traditional 4x4 axles. 


The frictional loss with our trucks' current unit hub bearing and axle shaft arrangement is "parasitic" drag.  To what degree full-floating wheel hubs and freewheeling hubs would improve mileage is uncertain, though.  The cost of the conversion is substantial and needs to be weighed.  Amortizing the cost in fuel savings might take a long time.


Dynatrac makes a quality conversion kit for our Dodge Ram trucks.  See the link below for details:





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Real quick, before I end up forgetting. I spent a long time talking to Cummins, he told me that in stock form my truck makes 440 torque @ 1600-2100, and 235 hp @ 2300-2700 rpm. Im still waiting to hear back from Smarty to figure out what their programmers will do.

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Interesting...Factory Chrysler "tuning" drops the torque peak rpm down to the lower end of that spectrum...Usually, de-tuning of an engine takes powertrain stamina into account.  The automatic transmission versions of these Ram truck diesels tend to get the lower performance ratings during these model years.  The 46RE, 47RE and 48RE transmissions are clearly not candidates for competition "puller" torque, and that's likely the reason for the de-tuning.


What's interesting is the potential horsepower and torque latent in these Cummins engines.  The basic paradigm with a turbocharged diesel is that the more fuel flowing into the engine, the more horsepower and torque.  Exhaust temp goes up commensurate to the amount of fuel getting shoved into the engine, so there's a limit to the power boost that the engine will tolerate.  For a Cummins ISB, this is all adjusted by the tune:  boost, injection timing and fuel flow.


Trade-offs are that de-tuned means less power but also less EGT.  Most likely in the case of light-duty trucks with diesel engines, the torque peak is also regulated to keep transmissions, drivelines and axles alive...



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Hi, I found this thread in a google search on Cummins MPG and joined this forum specifically to get in on this discussion.   I think I read everything above a couple times. The idea of running at peak torque makes so much sense I can't believe I have never seen it anywhere before.


I just bought a 2003 Dodge 3500 with NV5600.  The goal is to make it an efficient daily driver and trailer puller. We have a 7000 lb vintage camper so it's no problem for the truck.


I am most interested in fuel efficiency when running empty. My daily commute is 26 miles with just a few stops and turns on 55 MPH back roads.


I put on a set of used matching tires sized to the door sticker (235/80/17)

The MPG you describe above, is that calculated at fill up or based on the computer? So many people on these forums talk about mileage without making that clear.

Do you think switching to single wheels would help?  There is a lot of un-informed debate about that.

What about modifying the fan?   Again, in my reading I find a lot of misguided uninformed debate.   I know electric fans are theoretically more efficient, but the debate over making it work on a fully laden Dodge 3500 on a 7% grade  is too much for guys on a forum to unpack in a linear way so it's hard to get clarity.   In my case, I have no plan to get anywhere near this trucks  max capability so I can afford some degradation of performance  in change for efficiency.
If not an electric fan, are there thermostatically controlled clutch fans that perform better than OEM?


Hope to hear from you.








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First of all, thanks much for joining our discussion. The more folks who share their real world experiences, the better.  All of us will gain in the long run.

To begin, I've learned that the only valid mileage check is actual gallons used versus the miles traveled. The odometer must be accurate, and that should be checked separately from the speedometer. I emphasize this because we're all inclined to think that an accurate speedometer calibration automatically means an accurate odometer reading.  It doesn't always work that way. 


The easiest and least expensive odometer check is the official five-mile test on an interstate or highway that has accurate markers in place.  If you can hold the throttle (cruise control helps) steady at 60 mph, this can also be a quick speedometer check:  At 60 mph, each mile should be exactly one minute on your watch's second hand.


Once you have the odometer's accuracy confirmed, you can check fuel mileage by accurate fuel fill-ups.  The factory digital mileage meter can then be compared to real world gallons burned over X-number of miles.   


As for single drive versus dually, any mileage difference should reflect vehicle weight savings, and little more.  If there is a frictional loss issue with dual tires per side, I have not heard of significant gains with a single drive rear wheel setup.  In talking with many Dodge Ram 2500 and 3500 owners, both SRW and DRW (3500) trucks, fuel mileage does not seem to follow rear wheel layout with any consistency.  This would suggest that axle gearing and tire diameter, vehicle height and mass, accessories added, driving style and other factors hold more sway.  The reason for dual rear wheels is safety if you plan to load the bed heavily or pull a hefty conventional trailer or especially a larger fifth-wheel trailer.


Electric cooling fans are not a free ride, as they draw amperage from the electrical system and alternator.  Although the amp draw is not severe, as would be the case with an onboard frequency welder or other high-amperage draws, it is measurable with regard to engine load.  Before arguing that an electric fan eliminates the "parasitic power drain" of the factory mechanically driven fan, turn your attention to the CFM flow of each and the amperage load. 


Note: The engine driven fan can pull a huge amount of air through the radiator when the fan clutch couples.  It is only in recent years that the very best electric fans and shroud systems have approached this CFM flow. 


Personally, I prefer an engine-driven mechanical fan with a clutch unit.  As you hint, a thermostatic fan clutch can be improved upon, and I have used high performance aftermarket fan clutches to great advantage.  If you take this approach, be prepared to live with the sound of the fan coupling up at unanticipated times.  Most truck fan clutch thermostats or viscous fan couplers respond to hot air coming through the radiator core—rather than a specific engine coolant temperature.


The reassuring part of this process is to watch the coolant temperature drop immediately when the fan noise occurs.  For this to happen, the radiator and water pump must have sufficient flow rate in GPM/GPH, and the radiator must have enough tube and fin surface area to dissipate the maximum BTUs produced by the engine under full load. 


A great advantage of a fan clutch is its overrunning, freewheeling or uncoupling ability at higher rpm.  Without uncoupling, a fast spinning fan and a vehicle at increased road speed would be at odds with each other.  Ambient air rushing through the radiator core could get blocked by the high speed fan, and this obstacle to air flow would decrease cooling ability. 


An aftermarket electric fan is often either on or off, regardless of road speed or the volume or velocity of ambient air flow through the radiator.  The signal for an aftermarket electric fan is either coolant temp or radiator surface temp.  Once on, the fan spins at a fixed speed regardless of road speed.


My approach for maximum cooling has most often been an engine driven fan and a substantial shroud.  I've built oversized shrouds to capture the radiator's entire surface area heat, capturing and directing hot air toward the fan.  This eliminates hot air stalling and radiator hot spots.


Next, the heavy-duty fan clutch must be thermostatically controlled to come on at a temperature that will keep the lid on the engine's upsurge of heat under load.  This is even more critical with a turbo-diesel's boost and instant increases in exhaust temperatures.  I am comfortable with any power loss caused by a mechanically driven fan.  If I can keep the engine temperature under control, the gains far outweigh the fan drag! 


Here is just one example of a thermal type heavy-duty fan clutch: http://www.summitracing.com/search/product-line/derale-performance-fan-clutches/make/dodge/engine-size/5-9l-359/engine-family/cummins-diesel/clutch-style/heavy-duty-thermal.


Regarding quick heat surges, the factory's omission of a pyrometer on our Ram trucks is an area of concern.  I do intend to install a pyrometer (properly, and we can discuss the correct approach).  I plan to use the pyrometer to achieve maximum performance without shortening the engine's life.  While the tachometer keeps us in touch with the torque peak, a pyrometer lets us know when to back out of the throttle and stop stuffing fuel into the engine under boost.


From what you describe, and assuming you have 3.73 or 4.10 axle gears with the stock diameter tires, you should readily get 22 mpg at 55 mph.  Running empty when our Ram 3500 was strictly stock, I did even better than that (23-25 mpg) when I carefully followed the tachometer during upshifting and through each of the gears.


The easiest place to gain mileage is through driving technique.  Find your engine's "sweet spot" for fuel efficiency.  I'd suspect it's around 1600-1800 rpm on an '03 5.9L, that's at least a place to start.  Once you achieve your best mileage run and benchmark, stay within that rpm range whenever practical.  Straying from the sweet spot will cause an immediate drop in fuel mileage.


Lastly, there are the aftermarket improvements that can keep the engine alive and still boost performance and fuel efficiency.  In hindsight, I would say that the most significant impediment to my truck's fuel efficiency was not the added weight of accessories (though that's surely a factor) as much as the chassis/tire lift and the large winch bumper, which amplified the truck's exposed frontal area. 


The overall effect was an increase in drag coefficient.  I've actually gotten 25/26 mpg with this truck in its current profile (running empty) by simply catching enough tailwind to offset the frontal drag.  I did drive within the engine's sweet spot to achieve this fuel efficiency.


We can and will discuss this further.  There are always "trade-offs".  A lift kit and oversized tires can be much like a cabover camper or travel trailer:  Any of these factors create drag that often mimics a billboard getting pushed down the road!



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I think part of the advantage some people get from electric fans is simply settling for less cooling.  For vehicles that are not run at the limits of their capability eliminating the parisitc drag and replacing the cooling with a less capable fan will result in some gians, right?


What about dropping the final drive to 3.42?  I know, crazy, but follow my particular logic;


I know i have 3.73 gears now.  Where I live and where i would do my daily driving is pretty flat.  The NV5600 give me lots of choices to match RPM as needed, I could start using 1st and 2nd more.  


This is more truck than i need. It happend to be a very good deal, it's cosmetially bad and mechanically good (far as I can tell) But it's a lot of truck.    I would have chosen a 2500 or Ford F250 or perhaps a Diesel suburban. But  I wound up with this truck for just over $4K so if it works out I will modifiy to suit my needs and I don't care if i degrade it's overall capability.


My camper weighs 7200 lb.  With 3.73 gears this truck is rated at 13,800 lb.

With stock tires I beleive 3.42 gears would turn 1900 RPMs at 72 MPH.


I see people discussing the idea but I have not seen anyone post results.

Your thoughts?



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Hertfordnc...I followed a similar logic when 35" tires and a lift kit gave my wonderfully fuel thrifty '05 3500 an "overdriving" effect.  I harbored the illusion that the 3.73 gears with these new tires (similar to your current tires and 3.42 gearing) would possibly improve the mileage!


My first real experiment was pulling a toy hauler to the King of the Hammers Race at Johnson Valley, CA in 2012.  To that point, I had speedometer error and other challenges that kept me from seeing the light.  The trailer was the capper, though.  I had formerly pulled our XJ Cherokee on the car hauling trailer (open deck) to Moab and saw the mileage with the stock truck and 3.73 gears go from consistent 22-plus mpg running empty down to 17 mpg at interstate speeds (to 70 mph) and 6% grades while pulling the trailer.  I thought this was enough mileage drop until I tried the 35" tires with the 3.73s!


Towing the trailer loaded (8,500 pounds approximately) at mostly 55-60 mph, the mileage dropped to 10-12 mpg.  This did include the grades along U.S. 395 and on I-15, so we're certainly not talking about "flatland" towing.  Nonetheless, this was the worst mileage I have ever experienced with our Dodge Ram 3500.


I switched my thinking to trailer towing needs and our anticipated use of a travel trailer.  The gearing choices at the time were 4.10 or 4.56, nothing in between, with 4.10 being somewhat close to OE with the stock tires and 3.73 gears.  A true "correction" gearing would have been more like 4.30:1, which AAM axles did not offer at that time.  (They do now.)  So, I opted for 4.56 gears with the pledge that I would gear for 65-69 mph or approximately 2000-2100 rpm.  This rpm was slightly high for stock but consistent with the Hypertech Max Energy program I installed at the same time.


1980 rpm is the sweet spot now for keeping up with traffic and still getting reasonable fuel efficiency.  I pulled a 9,000 pound trailer over the I-8 grade from La Mesa, CA to the Anza-Borrego Desert this winter and managed 14-15 mpg, very respectable considering the load and a series of mountain passes.   


So, my guess is that 3.42 gears won't cut it, and you will be using all five gears on a regular basis!  Mileage will likely drop off.  In perspective, a friend just bought a new manual transmission 2014 Ram 2500 4x4, and it came with 3.42 gears and a six-speed.  He's thrilled so far, and we'll keep tabs on his success with this tall gearing.  Oversized tires would naturally be out of the question, even with the 6.7L engine.  The 6.7L engine has a catalytic converter and urea downstream system.  Arguably, the 6.7L will not "outperform" our 5.9L engines, which came stock with no cat or urea system.  My '05 HO rated 325 (some say 305) horsepower and 610 lb-ft torque in stock form.


I have considered a Gear Vendors overdrive behind the transfer case.  I have a 140.5" wheelbase and single piece rear driveshaft that could be readily shortened to accommodate the overdrive.  Here is a listing for our trucks:  https://www.gearvendors.com/ag4x4.html.  Though pricey, the overdrive/underdrive would be like going to taller gearing while retaining the current axle ratios as well.  Another distinct advantage is gear or torque splitting with this overdrive...Here is more Gear Vendors info for the Dodge Ram trucks:  https://www.gearvendors.com/d2wd4s.html.



Photo Courtesy of Gear Vendors


Some things to ponder, Hertfordnc!



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As a footnote, you benchmarked 1900 rpm at 72 mph as a possible target goal for fuel efficiency, Hertfordnc.  We're all familiar with "lugging" an engine, and under load and weight, that 1900 rpm could demand considerable turbo-boost to achieve 72 mph.  We also need to look at boost curves, since loading the engine triggers turbo boost and injector flow, which directly bears on fuel efficiency...A boost gauge would clarify the function of turbocharger boost under load.


Keep in mind that our engines are not just diesels, they are turbo-diesels that derive considerable power under boost.  This requires more fuel, though, as there's no such thing as a free ride.  The assumption is that a properly gated and meted boost will help performance and fuel efficiency if not abused.  To maintain proper air/fuel ratios under boost, however, does require more fuel to match the additional, compressed air.



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But aren't we talking about the same thing?   It was the use of the 3.42 gears on the newer trucks that got me thinking about this.    I'm not as co ncerned with towing mileage as I won't be towing that often. 


The other five gears would still give me plenty of choices.


What am  I missing?

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