6.7L Cummins Tires and Gearing

[Holt R]

I’m new to the forum but have already spent a little bit of time reading earlier articles and posts (many of them very informative).  My friend and I were having a discussion on gear ratios and bigger (35”) tires, specifically for a 2015 6.7l Cummins Ram 3500 (single rear wheel) with the Aisin transmission (stock gearing of 3.42).  Over the course of the discussion we had several unanswered questions about re-gearing for that truck and for bigger tires in general:

1. Why is a numerically larger gear ratio (like 4.10) referred to as “lower” gearing while a numerically smaller ratio (like 3.42) is referred to as “higher” gearing?  It seems counter intuitive.  I understand that the higher ratio has more gears on the ring gear and less on the pinion, which allows for better towing and slow-speed crawling performance.  Conversely, I understand that a lower ratio has less gearing so to speak but is better suited to highway efficiency.  Where do the “lower” and “higher” aspects come in? 
2. Can re-gearing to a numerically higher gear ratio help offset some of the mpg losses normally associated with bigger tire sizes?  I understand that some amount of efficiency degradation is unavoidable when switching to bigger tires.  But I’ve also heard that re-gearing can help put the transmission back into its optimal RPM band and also helps to reduce drivetrain strain caused by bigger tires.  Is that correct?  I only ask because I've seen a lot of Ram HD owners complain about significant mpg losses when making even a mild transition from stock tire sizes to 35's.
3. What is an accurate way to determine the ideal gear ratio for a given tire size?  I’ve seen an equation mentioned in several forums and youtube videos: Ideal Gearing = (New tire size * stock gearing)/(old tire size).  Going with that equation (assuming stock tire size of 33”), the ideal gear ratio for 35” tires seems to be ~ 3.63.  The closest conventional gear ratio is 3.73.  Would that be ideal for a multiple use (offroading, towing, highway cruising) diesel-equipped Ram 3500 wearing 35” tires?  What RPM band should we be aiming for with this new 6.7l Cummins?

I realize that much of this topic was discussed in an earlier post by Moses Ludel back in 2013 (5.9l Gearing Article).  His article was focused on a Ram with the 5.9L Cummins and the 48RE transmission.  The newer Rams’ stock gearing and engine outputs have changed a bit from those earlier powertrain setup’s, so I’m wondering if his scenario is directly applicable to what my friend and I are dealing with.

[Moses Ludel]

Hi, Holt, and welcome to the forums!  You've got a great truck and powertrain. Regarding your questions:

1) The term "lower gearing" is literal.  The reference is to gear ratios that would compare with a transmission gear set.  Since the axle is the "final drive" or gear, the reference is valid..Example:  If the original gearing is 3.42:1 ratio, installing a 4.10 ratio is like shifting the transmission to a lower gear.  Higher gearing (often called "taller gearing") would be the reverse:  If you have 4.10 gearing, the change to 3.42 ratio would be the same as shifting the transmission to a higher gear.   It's easier to think of this in terms of engine rpm.  Lower gearing raises the engine speed at a given road speed.  Higher axle gearing lowers the engine speed at that same road speed.  Truck talk...

2) Yes, assuming the tires are not so large that the vehicle's dynamics are upset badly; a higher body height means more wind resistance, and aerodynamics play a large role in the fuel efficiency equation.  Think of the axle gearing needed to "restore" the engine speed at a given road speed.  Let's use 60 mph as an example.  What was your engine speed with stock tires in overdrive at 60 mph?  Now that you have 35" tires with the stock axle gearing, the engine speed (rpm) is much lower (like an extra overdrive gear!).   To raise the rpm at 60 mph  (or any given road speed), you need to gear "lower", i.e. the 3.42 gears will need to be changed.  A lower gear ratio like 4.10:1 will do...Assuming that these owners all corrected the speedometers after gearing changes and fuel mileage checks, the reason for a drop in mpg with oversized tires and stock gearing is that you have increased the load on the engine.  Even though the engine is spinning slower, much like adding an extra overdrive, the loss of pulling power can impact fuel efficiency.  So can the aerodynamics change with 35" tires.

3) I like to use an engine rpm chart that takes tire diameter and axle gearing into account.  This site is very helpful:  http://www.csgnetwork.com/multirpmcalc.html.  I use the lower chart that involves the overdrive ratio.  In the case of the 31% overdrive on the 48RE, I use a final transmission gear ratio of 0.69.  You will use the overdrive ratio in the Aisin to figure the "Transmission Final Gear Ratio".  I run the numbers at 60 mph, 65 mph and 70 mph for the comparison axle gear ratios.  In your case, you will begin with 3.42:1 with stock tires.  This is your baseline.  Note the engine rpm at that road speed in overdrive.  Now use the available gearing options, which would be 3.73 or 4.10 in your case.  AAM offers these common ratio options.  3.92 might be available somewhere in the aftermarket.  

If you're somewhere between 3.73 and 4.10, consider your trailer towing and other uses for the truck.  Also consider the aerodynamic losses of the lifted truck with oversized tires.  Personally, I would go with the 4.10 ratio for 35" tires. 

Overall, with a Ram/Cummins and most other powertrains, to save fuel, slow down.  Aerodynamics become a bigger issue as vehicle speed increases...With my 5.9L ISB engine, 1900 rpm remains the ceiling for fuel efficiency.  My aftermarket software tune peaks the engine torque at 2,150 rpm.  I never "cruise" beyond the 2,150 rpm torque peak, and I do pay for the 2000-2150 rpm range with more fuel consumption.  With my 37" tires (true 36.5"-36.75") and 4.56:1 axle gearing, I'm at approximately 1900 rpm in overdrive at 65 mph.  Before lifting the truck and adding large diameter tires, the engine's "sweet spot" was 1600-1900 rpm.  Now, the most noticeable drop in fuel mileage is from aerodynamics and add-on accessories weight:  The faster I drive, the more fuel consumed...Period.  The largest single fuel efficiency factor is vehicle speed/resistance.  Once past 65-69 mph, fuel consumption is clearly on the rise!

Regarding my 48RE versus your Aisin transmission, the cruise fuel efficiency is still governed by the same factors.  Your overdrive ratio might be different than mine, but for mitigating the increased fuel consumption from bigger diameter tires, the concerns are still loss of power, wind resistance/aerodynamics and matching the engine's peak torque and fuel efficiency rpm to your desired cruise speed.

Moses (Ludel)

[Holt R]

On 4/6/2018 at 11:27 PM, Moses Ludel said:

3) I like to use an engine rpm chart that takes tire diameter and axle gearing into account.  This site is very helpful:  http://www.csgnetwork.com/multirpmcalc.html.  I use the lower chart that involves the overdrive ratio.  In the case of the 31% overdrive on the 48RE, I use a final transmission gear ratio of 0.69.  You will use the overdrive ratio in the Aisin to figure the "Transmission Final Gear Ratio".  I run the numbers at 60 mph, 65 mph and 70 mph for the comparison axle gear ratios.  In your case, you will begin with 3.42:1 with stock tires.  This is your baseline.  Note the engine rpm at that road speed in overdrive.  Now use the available gearing options, which would be 3.73 or 4.10 in your case.  AAM offers these common ratio options.  3.92 might be available somewhere in the aftermarket.  

If you're somewhere between 3.73 and 4.10, consider your trailer towing and other uses for the truck.  Also consider the aerodynamic losses of the lifted truck with oversized tires.  Personally, I would go with the 4.10 ratio for 35" tires. 

Overall, with a Ram/Cummins and most other powertrains, to save fuel, slow down.  Aerodynamics become a bigger issue as vehicle speed increases...With my 5.9L ISB engine, 1900 rpm remains the ceiling for fuel efficiency.  My aftermarket software tune peaks the engine torque at 2,150 rpm.  I never "cruise" beyond the 2,150 rpm torque peak, and I do pay for the 2000-2150 rpm range with more fuel consumption.  With my 37" tires (true 36.5"-36.75") and 4.56:1 axle gearing, I'm at approximately 1900 rpm in overdrive at 65 mph.  Before lifting the truck and adding large diameter tires, the engine's "sweet spot" was 1600-1900 rpm.  Now, the most noticeable drop in fuel mileage is from aerodynamics and add-on accessories weight:  The faster I drive, the more fuel consumed...Period.  The largest single fuel efficiency factor is vehicle speed/resistance.  Once past 65-69 mph, fuel consumption is clearly on the rise!

Regarding my 48RE versus your Aisin transmission, the cruise fuel efficiency is still governed by the same factors.  Your overdrive ratio might be different than mine, but for mitigating the increased fuel consumption from bigger diameter tires, the concerns are still loss of power, wind resistance/aerodynamics and matching the engine's peak torque and fuel efficiency rpm to your desired cruise speed.

Moses (Ludel)

 

Moses, thank you for the detailed answer.

I used the RPM calculator in the link you provided.  I first did the baseline calculation using stock (~33" tires) with 3.42 gears, and the Aisin's final drive of .63.  The resulting RPM for 65 mph (probably the ideal cruising speed for reasons you already noted) was 1426 RPM, which seems quite low for the 6.7l Cummins.  For curiosity's sake, I also tried 75 mph and got a RPM of 1645, which seems closer to the engine's peak torque sweet spot (1700 RPM).  I'm guessing the FCA engineers set up the gearing to allow for 75 mph at low RPM's?  Even if so, I do agree that with a big truck, aerodynamics will certainly degrade fuel efficiency at that speed, even if the engine RPM is at an efficient value.

 

Using that as the baseline, I did two more calculations:

1. 35" tires with 3.73 gears.  At 65 mph, the RPM was 1466; again low, but it seems close to OEM specs.  At 75 mph, the RPM was 1692, which again puts it very close to OEM specs and the peak torque RPM.
2. 35" tires with 4.10 gears. At 65 mph, the RPM was 1612 RPM.  At 75 mph, the RPM was 1860; this seems a bit higher than what I would want for long highway cruises, no?

The stock gearing RPM calculations kind of surprised me.  With stock tires and gearing, it almost seems as if you'd be lugging the engine at 65 mph.  But if my only concern is to put the engine back into OEM RPM ranges with 35" tires, the 3.73 gearing does seem to do that.  

I will also note that on this hypothetical truck we're discussing, there is a strong possibility of putting on a lot of aftermarket gear (bumpers, winches, auxiliary fuel, ect.) to the tune of 1k lbs.  If that was the case, I'd imagine 4.10's would make more sense for moving all that extra weight, in addition to any extra payload and trailers, correct?  If kept at a mostly stock base weight, would the 3.73's be a better choice?

[Moses Ludel]

Holt R...Thoughtful approach you're taking...See my comments below...Moses

On 4/8/2018 at 11:59 AM, Holt R said:

 

I used the RPM calculator in the link you provided.  I first did the baseline calculation using stock (~33" tires) with 3.42 gears, and the Aisin's final drive of .63.  The resulting RPM for 65 mph (probably the ideal cruising speed for reasons you already noted) was 1426 RPM, which seems quite low for the 6.7l Cummins.  For curiosity's sake, I also tried 75 mph and got a RPM of 1645, which seems closer to the engine's peak torque sweet spot (1700 RPM).  I'm guessing the FCA engineers set up the gearing to allow for 75 mph at low RPM's?  Even if so, I do agree that with a big truck, aerodynamics will certainly degrade fuel efficiency at that speed, even if the engine RPM is at an efficient value.

My guess, too.  Most want to roll a vehicle at 70-plus mph, and the 6.7L has a broad enough "high torque" range to handle 55-75 mph cruise speeds.  I would not drop below 1400 rpm for sustained towing.  A friend with 3.73 gears and stock tires changed to 4.10s to tolerate California's towing speed of 55 mph in overdrive.  For stock diameter tires, you would be opting for 3.73 or 4.10 gears to meet slower speed towing demands...or dropping down a transmission gear to cruise in California.

Using that as the baseline, I did two more calculations:

1. 35" tires with 3.73 gears.  At 65 mph, the RPM was 1466; again low, but it seems close to OEM specs.  At 75 mph, the RPM was 1692, which again puts it very close to OEM specs and the peak torque RPM.
2. 35" tires with 4.10 gears. At 65 mph, the RPM was 1612 RPM.  At 75 mph, the RPM was 1860; this seems a bit higher than what I would want for long highway cruises, no?

If you're trailering a lot, say an 8,400# loaded travel trailer like ours, I would do the 4.10s.  If you run empty the bulk of the time, the 3.73s would be an improvement over the stock 3.42s.  Again, keep in mind that OEM restoration is usually not enough here, you have a lifted truck with more wind resistance.  Stock equivalent would not be low enough gearing for any kind of towing.

The stock gearing RPM calculations kind of surprised me.  With stock tires and gearing, it almost seems as if you'd be lugging the engine at 65 mph.  But if my only concern is to put the engine back into OEM RPM ranges with 35" tires, the 3.73 gearing does seem to do that.

You're right.  This is tall gearing.  FCA's goal is fuel efficiency from a larger engine than the 5.9L.  6.7L gearing has moved toward the tall side!  The 6.7L engine can handle this, but in a commercial application, Cummins recommends 2050 rpm at a 65 mph cruise speed for a commercial vehicle to 50,000# GVCW on-highway.  Also, Cummins does not want the commercial ISB engine to drop below 1800 rpm at cruise.  (This is the Economy setting, not for maximum performance, which would raise the engine rpm even higher.)  You can see that FCA targets lighter loads.  From experience, if you want fuel economy, I would not use Cummins' higher engine rpm speeds at cruise unless you're pulling a multi-car hauling gooseneck trailer or a nine-horse equestrian trailer.

Note:  In my view, hefty loads approaching anything close to commercial limits with a stock Ram truck's gearing will inevitably lead to automatic transmission failure.  Your Aisin transmission is considered the best automatic transmission offered to date, and it has limits, too...The Cummins 6.7L ISB diesel engine can withstand severe punishment and has a broad, high torque output.  The weak link would be the transmission.

I will also note that on this hypothetical truck we're discussing, there is a strong possibility of putting on a lot of aftermarket gear (bumpers, winches, auxiliary fuel, ect.) to the tune of 1k lbs.  If that was the case, I'd imagine 4.10's would make more sense for moving all that extra weight, in addition to any extra payload and trailers, correct?  If kept at a mostly stock base weight, would the 3.73's be a better choice?

I did these add-ons with our 2005 Ram 3500 Quad-Cab 4x4.  For this added weight, even when not trailering, 4.10s should be your only choice:  Fuel mileage will be what it is.  The 3.73s are out of the question, and 3.73s in California would have you towing out of overdrive to prevent powertrain damage! 

I ran 4.56s with 35" tires and 31% overdrive.  Gearing was too low, the engine ran faster than necessary and burned too much fuel.  I opted for 37" tires to get engine speed into the right rpm range at a 65-70 mph cruise speed in 0.69 ratio (48RE) overdrive.  You have more torque than my 5.9L, so dropping engine rpm for fuel efficiency is practical:  4.10s make sense.  At 75 mph, you will get compromised mileage from the aerodynamics;  a trailer above cab height will obviously increase that aerodynamic load further.  After your gearing change, play with the trailer towing speeds.  See where the fuel mileage goes over the cliff.   I'm now geared for optimal trailering at 65 mph in overdrive.  I slow down and kick out of overdrive on 6% or steeper grades under a heavy load.  Overdrive is the weakest and most vulnerable gear in the transmission.

My maximum engine speed for all but passing emergencies is 2,150 rpm, the peak torque point for the 5.9L's MaxEnergy tune from Hypertech...

Moses