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Oversized Tires Can Create "Too Much Overdrive!"

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#1 Moses Ludel

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Posted 02 July 2013 - 05:24 PM

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.4wdmechan...r-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.4wdmechan...le-Rebuild.html [detailed article with how-to steps in color photos]


http://www.4wdmechan...le-Rebuild.html [an overview that works in conjunction with the 11.5" AAM axle rebuild article]



#2 Megatron


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Posted 16 July 2013 - 07:28 AM

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

If you think its expensive for a professional to do it, wait until you see what it cost for an amateur to do it... 

#3 Moses Ludel

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Posted 16 July 2013 - 12:23 PM

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...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.jeep4x4ce...om/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!



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