Oversized Tires Can Create "Too Much Overdrive!"

[Moses Ludel]

The exchange with Megatron around his 48RE shudder at take-off reminded me of installation of oversized tires on our Ram 3500 4WD truck.  Prior to installing the 4" lift and oversized tires, the truck had achieved great fuel mileage as a stone stock vehicle.

 

I was thrilled with the Ram's fuel mileage and performance from new (October 2004) until the summer of 2011.  Then it was time to bring the truck to the standard that readers and sponsors like—lifted, accessorized and sporting oversize tires!  (See the Ram truck build up at the magazine: http://www.4wdmechanix.com/2005-Dodge-Ram-3500-Major-Makeover.html.) 

 

Stock gearing was 3.73:1, and with a Cummins 5.9L ISB engine, that meant cruising between 1,600 and 1,900 rpm most of the time.  This worked perfectly for fuel efficiency, much to Chrysler's engineering credit.  My expectation, considering the extraordinary low-end torque of the H.O. diesel, was that oversized tires would have little impact on the fuel mileage—in fact, I even speculated that the mileage would improve, since the engine could stay in the 1,600 to 1,700 rpm range at interstate speeds!  Peak torque for this engine is at 1,600 rpm, optimal for fuel efficiency.

 

Boy, was I in for a surprise!  Trips to Chico, CA for the Transfer Flow fuel tank installation and the subsequent run to the 2011 Off-Road Expo at Pomona gave a hint.  Mileage seemed stagnant and, if anything, off its usual peaks.  I attributed the unimpressive mileage to mountainous roads and higher cruising speeds, but the true problem reared itself when I towed a 7,500# toy hauler trailer to the 2012 King of the Hammers Race at Johnson Valley, CA.  The trip was to film the races and interview celebrities like Shannon Campbell that week.  (You'll find this coverage and more at the 4WD Rock Crawling & Racing Channel on the magazine website.)

 

Trailer in tow to places like Moab, UT, the truck had achieved 17 mpg at interstate speeds and 6% grades with the stock diameter tires (under 32" diameter).  Now, with 35" tires, the mileage with the same weight trailer in tow, adhering to California's trailering speed of 55 mph, the mileage plummeted to 12-13 mpg!  Before all of the modifications and weighty accessories, at 55 mph with the stock tires and gearing, trailer towing would have netted 19-20 mpg!

 

Oversized tires with stock gearing creates an additional "overdriving effect".  Sometimes this is advantageous, but in the case of our '05 Dodge Ram 3500 4x4 Quad Cab, the combination of 1,350 pounds of new accessories and auxiliary fuel, plus the 35" tires, made the stock 3.73:1 gearing unacceptable.

 

The change to 4.56:1 gearing has bumped fuel efficiency back to a peak of 21-23 mpg (unloaded, full fuel capacity, no trailer in tow)—if I keep speed at or below 65 mph in overdrive.  While a direct correction for the tire size and axle gearing would have been 4.10:1, I knew that the added weight of accessories and auxiliary fuel, plus the increased drag from the lift, would make "stock" gearing no longer practical.

 

With the 4.56:1 gears, I do "pay for it" in extra fuel consumption when driving above 65 mph.  It acts like a linear thing:  The fuel mileage drops with each mph increase in speed!  Had I planned on driving over 65 in overdrive most of the time, without a trailer in tow, I would have opted for the 4.10:1 gears.  We do plan to pull trailers with a GVWR under 10,000#, so the 4.56:1 gears are optimal, and mileage is good—if I keep my foot out of the throttle!

 

If you own a Ram 2500 or 3500 HD 4WD pickup like ours, a suspension lift and oversized tires will likely demand ring-and-pinion gear changes.  I cover the 11.5" and 9.25" AAM axle re-gearing at the magazine site:

 

http://www.4wdmechanix.com/How-to-AAM-11.5-Axle-Rebuild.html [detailed article with how-to steps in color photos]

 

http://www.4wdmechanix.com/HD-Video-How-to-AAM-9.25-Axle-Rebuild.html [an overview that works in conjunction with the 11.5" AAM axle rebuild article]

 

Moses

[Megatron]

I have a couple questions: How do you determine the best gear ratio for your truck and its tire size?  

 

I've found a few sizing/gearing charts at various places on the web but I figured it was more complex than that. I know that no 2 engine designs (example: diesels vs. gas) are equal in power and efficiency range. Plus with countless variations to add to it like modifications to your engine/transmission and primary driving locations (mountains or flatlands etc.), how do you decide? Also, at what point do you consider towing a trailer part of the equation? I plan to haul a trailer, but maybe 2-5 times a year (a trailer with real weight on it that is).   

 

I pick up on the fact its about getting the engine in its optimal operating range, but at what MPH do you really strive for? I live in an area where 70 mph is what the man says, but I push that by 5 mph everywhere I go. I would say I'm 50/50 on highway vs. in town.

 

Plus (and this may be way off topic and too complicated to answer) but after doing things to your engine in ways of cams or compound turbos and/or electronics, how does one really find out where the new peak efficiency range is without some extensive dyno/road testing?

 

What is your opinion on it is better to build my truck for highway and pay the price in town or build a town truck and pay for it on the highway?

 

  As always, thanks for your shared knowledge and input. Sorry if my replies are all over the board, just I have so many questions about everything and I'm not great at separating them based on topics. lol

[Moses Ludel]

Your questions make perfect sense and are welcome!  My opening volley on this topic reflects my perspective, and I'll elaborate in more detail...

 

Best axle gear ratio is a somewhat loaded question and does consider tire diameter.  I wrote magazine Q&A columns for fifteen years in the 4WD Jeep, 4x4 truck and muscle car fields.  Gearing was a constant question with tangible solutions.  In the late '80s, I got a nifty calculator program from Wolverine, intended for camshaft selection, tire sizing and the right gearing choice.  Though DOS based and no longer accessible (thanks to Microsoft 7 and up), that program provided a great calculator utility.

 

Fortunately, there are similar calculators for axle gearing and tire diameter now online.  Here is one downloadable software calculator that you can trust.  It's directly from Cummins and includes your '06 Cummins 5.9L ISB Dodge Ram 4x4 Mega Cab—directly from the source: http://www.powerspec.cummins.com/site/home/index.html.  You'll find a great calculator for Cummins' commercial diesel engines.  Note: I use the earlier version 4.2.4 software for the 5.9L ISB engine; the latest version only covers the 6.7L ISB, although this engine is similar in most ways for this calculation.

 

I use this program, and it's very cool, addressing tire diameter, axle gearing (include the 0.69 ratio for overdrive on the 48RE) and engine rpm.  Cummins has a recommendation for optimal commercial performance, which is actually close to the weight and load factors you and I anticipate: occasional towing, hefty accessorizing, the lift (which makes our trucks like pushing a billboard down the road) and both town and, primarily in my case, highway use. 

 

There's one caveat when using the PowerSpec program: The program aims at commercial haulers.  Cummins wants to see a higher rpm (2100-2400) than I prefer.  My approach for maximum fuel efficiency with a modified truck weight similar to ours (9000-9300 pounds curb weight) and the 5.9L H.O. ISB engine is in the neighborhood of 1900-2000 rpm at highway cruise.  I see a prompt and notable drop in fuel mileage for engine speeds over 2000 rpm—although power remains great between fuel stops!

 

For gasoline powered trucks or a trail Jeep, here's a quick reference chart that you'll find useful: http://www.jeep4x4center.com/calculators/.  The chart's baseline is 65 mph with a 1:1 high gear ratio.  You need to factor the overdrive ratio into the final engine speeds at cruise.  Multiply the rpm times 0.069 for your 48RE transmission.  We have a significant 31% overdrive. 

 

There are other calculators online that allow for plugging in the overdrive ratio variable (0.69, 0.75, 0.85, etc.).  Here is one that offers space for the overdrive ratio in the equation: http://www.4lo.com/4LoCalc.htm.

 

As for picking between 4.10, 4.56 and 4.88 gears (the only ring-and-pinion options for the AAM axles) in your specific truck, the answer is subjective.  I'll tackle the question, though, and will share what I believe each of these ratios will deliver with your 37" tires:

 

1) 4.10:1 would nearly restore your gearing to the OEM level with stock diameter tires; still some overdriving effect, more like OEM tires with 3.55 gears instead of 3.73:1.  I would definitely not use this gearing for trailer pulling.  Town traffic would be sluggish, too.

 

2) 4.56:1 would be quite livable, all around.  Acceleration would be slightly better than OEM tires and gearing at the OEM curb weight and normal cab height.  This is my gearing now for 35" tires, and I know that a change to 37" would be feasible.  I'm spinning the engine close to 2,000 rpm at 65 mph...To drive at 75, like you want, the Cummins calculator actually thinks the engine should spin faster—I think it would be right on for performance and reasonable fuel efficiency at this load.  (I'm figuring 37" tires at 560 revs per mile.  Is this correct for your tires?  Confirm for calculations.)  560 revs per mile and 4.56 gears in overdrive cruise mode would put your engine at 2202 rpm for 75 mph and 1909 rpm at 65 mph.  This would deliver peak fuel efficiency at 65 mph and decent performance at 75 mph, a satisfying all around choice for your truck with the 5.9L ISB engine.

 

3) 4.88:1 would be okay if you trailer all the time and would like to hold speed at 65-70 mph while towing.  (You could push to 70 mph if fuel costs do not sway your thinking.)  Acceleration in stop-and-go would be impressive, the load on the engine and 48RE transmission would be less.  Extra piston travel per mile could reduce lifespan of the 5.9L engine; however, the reduced load would likely offset this...If I had a 9-horse trailer, this would be my gearing, holding the truck to 65 mph and keeping the engine and horses happy!

 

As for target mph on the highway, that's a subjective question, too.  You have a plan for 70-75 mph at cruise, and with a trailer, that would keep the engine at Cummins' recommended 2100-2400 rpm range in overdrive.  While I repeatedly emphasize the 1600-1900 rpm "sweet spot" for fuel efficiency, there is the realistic lugging factor that places the engine under stress below 1900 rpm when toting severe loads.

 

So, I think you'll be happiest with 4.56:1 gearing at 37" diameter tires.  I can feel with our truck that it's doing just what I wanted: Delivering trailer pulling torque, adequate horsepower and peak fuel efficiency at 65 mph.  4.10:1 would have restored the tire/gearing to stock, but as I've noted, this truck is way too heavy and tall at curb (unloaded with fuel in auxiliary tank) to survive on stock equivalent gearing.  I've equated my modified truck to pulling a tent trailer—all of the time!

 

As for highway versus town driving, you need to consider both.  Town driving is getting the mass rolling.  Highway is keeping that mass rolling.  Both impact fuel efficiency and loads on the engine.  Again, the 4.56:1 will enable town driving without taxing the powertrain much.  4.88:1 would make town driving easier, especially when moving a big trailer from a dead stop; however, the highway cruise engine speeds you plan would make 4.88:1 wasteful on fuel.  (2,357 rpm at 75 mph in overdrive is near peak rpm for Cummins commercial use recommendations.)

 

You're right about dyne tests to confirm power curves on a dramatically altered engine with camshaft modifications, fuel timing changes, added turbo boost and other alterations.  Let me start by saying "chips" will not dramatically alter the torque rise on a diesel engine.  Tuning measures will unleash suppressed power, but the curve shape will be similar.

 

Camshaft, compression or turbo mods are another story, as changes here can move the power curve around.  Although everyone seems horsepower fixated, diesel power is all about the quick torque rise and peak at a lower rpm.  I believe the Cummins ISB engine has the edge here, a lower rpm, traditional diesel design suited for medium duty truck use and patterned after commercial truck and off-highway equipment performance.

 

That said, drive your Cummins diesel accordingly.  I have "redlined" our truck's engine to its advertised 3,400 rpm peak on less than a half-dozen occasions in 121K miles.  Redline is pointless when the engine's torque peak is at 1,600 rpm.  2,400 rpm should be a sensible peak, maybe 2,800 rpm on a lengthy grade with our trailer in tow—and certainly not for a sustained period.

 

Note: Gale Banks and I met after the Off-Road Expo in 2011.  I visited Banks Power at Azusa, and we talked.  Gale is a Duramax diesel dealer and strong advocate, and his descriptive for the Cummins ISB engine was, "We blow the cylinder head off the block on those engines!"  They do when drag racing with engines built for extreme output and competition—or even when running these engines "to destruction", presumably at redline for sustained periods.  I have known Gale for thirty years, and he is expert at high performance and race engineering.  By contrast, I served an apprenticeship with the Operating Engineers Union in the mid-'seventies, and we worked large highway construction jobs.  If we ran a 1693 Cat inline six off-highway equipment engine much over 1,500 rpm, we were in jeopardy of losing our job.  These engines peaked horsepower below 2000 rpm and peaked their torque just off-idle!

 

Trust this helps.  It's really not that complicated once you establish a firm goal.  Even with the four-speed automatic, I've gotten as good or better fuel efficiency with the 48RE as others with the NV5600 6-speed manual overdrive transmission.  We get the added advantage of a torque multiplying converter, too!  

 

I can assure owners with the manual transmission, Cummins engine and the right gearing that 22-25 mpg fuel efficiency is very attainable on the highway under light loads and at normal cruising speeds...It all comes down to driving technique.  Running empty, my upshift points would be between 1,400 and 1,600 rpm per gear.

 

Trust this helps and thanks for posing these questions!

 

Moses

[wastintime]

I found your power steering and linkage discussions to be vital to my repairs. Everyone wanted to replace ball joints and things not needed. New tie rods and power steering box and its back to new. Thanks for the information.

 

So I am in the middle of a tire sizing dilemma. Can you help with how much bigger than stock 265/70/17 tires I can get away with? I am running 285/75/17 KM2 on my 06 2500 now and mileage while towing is sub par as you describe. It is in the 20 mpg range at 70-75 MPH empty.

I am only pulling my 25ft toyhauler to camp but it drops way down to 12 ish.

 

So is there another option? If I run a programmer or injectors etc can I keep my big tires and get my mileage back?

 

I should add that I have a 2000 dodge also with 4.10's so I could run big tires and that truck does about 16-18 at 65. So I try to keep RPM's down but it still is not right. I just want to run slightly bigger tires and towing with o.d. off is to much RPM for traveling.

 

I  can not get the cummins program to load up.

 

Truck is stock other than big tires, FYI.  Thanks

[Moses Ludel]

wastintime...Welcome to the forums, glad the information on steering linkage and the box helped.

 

First off on the tire size change:  Did you calibrate the speedometer to match the new tire size?  Are we talking about an accurate 20 mpg at 70-75 and 12 when pulling?  When I ran the 265/70s with 3.73 gearing and empty, the typical best mileage was 24 at 65-69 mph.  When I went to a steady 75, mileage would drop to around 22.  Here is my calibration video:  http://www.4wdmechanix.com/How-to-Dodge-Ram-Speedometer-Calibration.html.  Here's how to check error quickly:  http://www.4wdmechanix.com/How-to-Measuring-Speedometer-Error.html. 

 

What is the rated diameter of the 295/75/17 tires with your wheel width?  The 265s should have been around 31.9" if OEM type tires.  Your 295/75s are likely in the neighborhood of 34.5".  This is a fairly large bump if you have 4.10 gears and a manual transmission.

 

I did the reprogram with Hypertech and would readily acknowledge the performance gain.  Fuel efficiency gains were marginal, although it did not get worse.  The tune gives me peak torque by 2100 rpm instead of the rated 1600 stock.  Here're my findings: http://www.4wdmechanix.com/Hypertech-Max-Energy-Power-Programmers-for-Jeep-4.0L-and-Dodge-Cummins.html.

 

The only thing that will assuredly get your mileage back is a gearing change.  I have the 48RE automatic, and the closest correction for my 34.6" diameter tires would be 4.10 gears.  I opted for 4.56:1.  (4.30:1 was not available from AAM at the time, I understand that it is now.)  My tires are 34.6" (rated as a 35") diameter.  I went a bit far with the gearing—intentionally.  The plan is more trailer pulling and a 65 mph cruise speed peak when I can keep a lid on it.  The fuel mileage penalty is linear beyond 65 mph with these gears.  4.10s would have sufficed were it not for the hefty accessories, winch bumper and auxiliary fuel tank, etc.  I'm guessing curb weight "empty" (auxiliary fuel tank and fuel included) is around 8800-8900 pounds.  I need to weigh the truck and confirm...Had the 4.30:1 gears been available, I would have picked them. 

 

I'm currently planning a true 36" diameter tire change.  That would provide a compromise between the 4.10 and 4.56 ratios with the current 4.56:1 ring-and-pinions.  I'm geared a bit lower than I need, and like in your case, we're running empty a lot more than trailering.  With that moderate change (1.4" diameter), I would expect a true 21-23 mpg at 70 mph on a flat road.  If you decide to do the 11.5" and 9.25" AAM ring-and-pinion gear change out yourself, my video coverage at Vimeo on Demand would be useful:  http://www.vimeo.com/ondemand/aamaxlerebuild.

 

If you're referring to the link to Cummins PowerSpec, try this:  http://cumminsengines.com/powerspec. 

 

Trust this is helpful, pleased to take the discussion further...

 

Moses 

[wastintime]

I may have caused confusion, I have the "06" in question and a "00" 1ton dually. Both have KM2 tires, both have less than stellar mileage figures. I left out the specifics but its a stock truck other than the tires that are specked out at 34.3" but really about 33 1/4" installed on stock rims. I did not re calibrate or chip, etc.

 

I have 3.73 gears with an auto and it is fine other than towing through CA, I get poor mileage. The issue is it is to much rpm in 3rd while towing and to low of rpm in O.D.

 

I did run into a very detailed discussion of yours after I posted this, causing more thought  about size and width and rolling resistance along with rpm facts from cummins, but I still need to decide on:

 

#1 swap to smaller tires -side note about mudders vs all terrain and rolling resistance

#2 install a chip hoping to move the torque curve and shift points to accommodate the change or

#3 live with it. Gear change or "gear vendor" is a lot of effort for some looks .

 

Will a programmer get me by or do I need to go back to stock until I can change gears or get a gear vendor?

I would like to keep the big tires and get by with a programmer to fix the issues above.  I like the lower rpm with the bigger tires but I can see that its an issue.

[Moses Ludel]

Wastintime...You do have a good bump in tire diameter, much like my '05 3500, which now has 34.6" diameter tires.  I really lost fuel mileage with these tires and 3.73 gearing.  That was also before the chip change, though.  The later programming/tuning change was a major performance gain.  In discussions, Hypertech believed that fuel mileage would go up considerably with the tune, but since I installed the program after the change to 4.56:1 gearing, I cannot confirm a significant gain.

 

What I do know is how hypersensitive the Cummins ISB 5.9L engines can be to rpm and load.  If you go to Cummins PowerSpec, commercial vehicles (to 50K GVCW) use lower gearing like 4.10:1 with our original size tires.  The chart would have 4.56 for tire diameters beyond this.  (Cummins thinks the ISB engines should spin 2150-2400 rpm at the highway cruise speed.  That would take our fuel mileage into the dirt, but we're not lugging the commercial loads of a medium duty truck.)  That was my motivation for the 4.56:1 change, as Cummins does know these engines.  I'm not, however, constantly pulling a nine horse trailer that would rival commercial use.

 

So, if the oversized tires are important, a direct restoration in gearing would be 4.10 in your case.  If you plan to tow regularly and accept that towing does create a different fuel mileage dynamic, then 4.30:1 (if available) might be a good choice.  In any case, after careful monitoring and experience, I do accept that my truck will never see 24 mpg consistently again.  This is due to the increase in curb weight, the lift and aerodynamics losses, and the friction of the tires, which you hint about when questioning tread patterns.

 

Ironic that you mention the California towing scenario.  My first inkling that the 3.73:1 gears would not work with the oversized tires was a trip from the Reno Area to King of the Hammers (Johnson Valley) with a Vortex toy hauler in tow.  I thought that my truck, with its historic track record for fuel efficiency plus the "overdriving effect" of the big tires, would get better mileage than earlier tows.  Years prior, on a trip to Moab with a virtually stock, non-accessorized truck and the XJ Cherokee on our car hauling flatbed trailer, we towed 70 mph with the cruise set, including 6% grades, and netted 17 mpg.  To my great disappointment, the Johnson Valley trip with big tires and 3.73:1 gearing netted a whopping 12-13 mpg at best!

 

We're bucking physics:  There are trade-offs for every change made to our trucks.  Lift kits and oversized tires, add-on accessories, removing the air dam up front, take your pick, each takes a toll.  Realistically, the only way you'll win here is apples for apples.  I believe the engine programming change was very worthwhile if for no other reason than pushing the increased torque peak up the rpm scale to a realistic 2,000-2,100 rpm range.  For your cruise speeds and towing habits, this could help.

 

I would suggest the use of the Hypertech Max Energy program.  Try it first with your current tires and gearing.  (I'd really like to know your results!)  One of the best inducements for the Hypertech Max Energy program was its promised "tow friendly" tuning.  Although I would still like to add a pyrometer to confirm load and heat with this program, I've experienced no negative issues to date.  Note that I am very engine and transmission protective while towing and watch my engine coolant temp gauge for any indication of extra loads.  I am always driving for fuel efficiency, engine protection and to take a load off the 48RE transmission.  This latter concern is important.  The lower 4.56:1 gearing substantially reduced load on the 48RE.  Oversized tires with 3.73 stock gearing has the opposite effect.

 

BFG and other tire producers do rate tires for friction and economy.  As a rule, mud tire treads are not the best pick for economy and rolling resistance.  On your point #2, you currently are getting an "overdrive" effect with the big tires.  If anything, the wrong tuner/programmer might move the torque peak in the wrong direction!  This is not the case for the Hypertech Max Energy tune, however.  The torque at 1,600 rpm (stock torque peak rpm) is as good with the Max Energy program as with the stock programming.  (See the dynamometer run charts at my article on the Hypertech product:  http://www.4wdmechan...ge-Cummins.html.)

 

I'm without bias when it comes to helping owners get the best mileage possible.  There are several variables involved.  Let's work through this for your driving and towing situation. 

 

Moses

[wastintime]

So in theory the torque is still good on the dyno down low and more all around, then I may get away with it. Went to pismo and surprise we went through lots of fuel but guys hauling bigger loads did just as good so I know I have to fix this. My relatives have a 2 ton dodge and it gets terrible mileage with the low gears, lots of power though.

 

I want to run a programmer that will provide gauges like an edge or all the others, any suggestions. I don't want to trust anything that I can't watch the pyro with.

 

Any way it seems like this motor should take a taller tire easier and that more torque might even things out. I also felt like it would be better for flat out highway runs but didn't watch it much before the tire swap.

 

As another little bit of talk. I run a lot of different equipment on and off highway and it is all running higher rpm motors and hotter temps now due to emissions I am told. We run 500 hp MercedesCat/? motors up to 21-2200 rpm all day at full load and expect no issues, crazy.

 

Thanks

[Moses Ludel]

The dyne results are enough to warrant the programmer tune.  You have the option/settings for a mild tune, more aggressive tune and even stock.  An added benefit is the ability to calibrate your speedometer for the tire diameter.  All in all, the software fix is the least invasive and will serve you well over time.  I'm running the full performance mode and not pushing the throttle, which is very responsive when you do—at the cost of fuel consumption.  You will be impressed with the performance boost, very clearly Chrysler left a lot of power switched off.  In part, this is due to the 48RE transmission, so beware.  Adding power does place more load on a transmission noted for issues when stressed.

 

I do like your pyrometer approach, wasn't aware that Edge offered that option.  A standalone pyrometer kit is expensive but the best barometer for diesel preservation.  When installing the pyrometer, there is a very specific way to tap into the exhaust manifold and avoid damaging the turbo.  You want true engine exit exhaust temperature here.

 

The programming will put additional power into the equation.  I'm curious about the fuel efficiency gains with the oversized tires and stock axle gearing.  The Cummins ISB turbo engine is not a "high speed diesel".  I am a former heavy equipment operator (Local 3, Operating Engineers in the '70s) from the 1693 Cat era.  In off-highway tune, those 893 cubic inch, inline six cylinder off-highway engines produced and peaked 1090 ft. lbs. torque at 1000 rpm, with a governed maximum speed around 1700 rpm.  Tip in the pedal, maximum torque on tap.  In off-road equipment, they pulled and dragged like a locomotive with that kind of torque.  Off-highway emissions were a non-issue at the time.

 

The basic Cummins ISB design is from the same era as these Cat engines.  It behaves more like a medium duty truck engine in its Dodge Ram form.  The major gain with this engine over the Powerstroke/Navistar and the Duramax is the quick torque rise.  Here, an 8-speed transmission, or at least 6, would be good.  If I had my choice all over again, I would have gone with the manual 6-speed on this chassis.  Were it not for the considerable effort involved, I'd consider a changeover.  This is also my rationale about the Gear Vendors split shifting, although that gets busy and, as you note, would take some time to cost amortize.  My personally imposed rpm ceiling is 2400 rpm, though there have been times when I've pushed this engine to 2900-3000.  Higher rpm is possible but not rational.

 

Note: Discussing mileage, I have a friend with an '04 Ram 2500 Cummins.  Shifting at 2,400 rpm per gear (manual transmission) like a gasoline engine, he never saw better than 18 mpg with this truck.  I suggested the Hypertech programmer to make better power and efficiency at a higher rpm.

 

I am very interested in the results you get with your programmer pick.  Before the Hypertech, I was resistant to the idea, since the install, I'm very much in favor of a tune improvement here.  Let us know your programmer choice and the results!  I'm most curious to see whether the 3.73 gears and oversized tire diameter will work together.  Re-calibrating the speedo will help put the package into perspective.

 

Moses

[wastintime]

So I think I have wasted way to much time in analysis paralysis. Hypertech has a 30 day guarantee and they and the others all provide about the same boost without the gauges. The other programmers with the gauges give more power but my truck is plenty powerful now with the tires, etc as is.  

So I ordered the programmer and will use some old tires for now to test things out.

 

My theory is that I have power to go 70-75 towing now and empty its fine. I want to stay under 2000 RPM so I want to keep the big tires. Turns out that size BFG is 33.8" tall so they are big but the motor turns them fine.

I am planning to go down to a 275/70 or 275/75=32.2 or 33 and little less aggressive tread. I want to go 285/70 but its iffy if that will still be to tall a tire.

 

The problem is my tires are shot and all of this takes tires and time to figure out. So what I really wanted was someone in internet land to chime in and say "I did this and it was great" saving me the hassle.

 

I feel with a bit more torque and tuning from the chip, that a small bump in tire size will be tolerated, fuel wise and the tires are cheaper so I can absorb the programmer into the cost savings.

 

I ran some tire sizes through the calculator to correct for size vs stock:

275/70=32.2"=3.68 gearing 33 RPM difference

285/70=32.7"=3.                  62

275/75=33.2"=3.                  98

285/75=33.8"=3.40            134

It seems like the RPM difference was more with the new tires but not sure. I still can't believe that this little bit of difference is that big in the real world but....

I just cant stand the puny stock tires, any thoughts? ANYONE  

[Moses Ludel]

Wastintime...I like your approach here, heading back in the direction of OEM tire diameter (31.9"), keeping rpm under 2000 at your intended cruise speed.  Let me know what you think of the Hypertech programmer, I'm continually impressed with the performance gains.  Fuel efficiency is now determined by my right foot.

 

Moses