Ram 6.7L 4500: How can I drop engine speed for fuel efficiency?

[w ranch]

I have a 2013 Ram 4500 6.7L.  I only use this truck to pull a horse trailer at 17k pounds max weight.  The truck is all stock cab and chassis model with G56 six speed 410 gears.  At 65 mph rpm is at approximately 2100 rpm.  Is there a way to lower the rpm in 6th gear?

 

[Moses Ludel]

w ranch...Thanks for joining us at the forums...Is your Ram 4500 a 4x4 or 2WD?  Max gross combination weight, the truck and trailer, or a 17K pound trailer?

Your only methods for dropping rpm at 65 mph would be 3.73 axle gears (if available) or oversized tires.  Your overdrive in 6th gear of the G56 is 0.74:1 or 26%, and available original axle ratios are 4.44s and your 4.10s.  You have the taller gears available from the factory.

As an option to 3.73 gears, there may be larger diameter tires that will clear the truck's wheel wells and drop that rpm some.  I recently added 0.95" radius (1.90" diameter) to our Ram 3500 tires and dropped rpm at 65 mph from 1986 to 1883 rpm.  See the video I did and click on the fuel efficiency PDF at the bottom of the article:

http://www.4wdmechanix.com/selecting-tires-and-wheels-for-optimal-handling-gearing-and-fuel-efficiency/

I have 4.56 gears and a 0.69 ratio or 31% overdrive.  36.5" tires puts me at 1883 rpm at 65 mph for trailering, where I cap my speed at 65 mph...Your current tires are 225/70R19.5 size, the axle is 4.10 with a 0.74 ratio or 26% 6th gear overdrive.  The principle is the same, but you will not need 36.5" tires to get your engine speed below 2000 rpm! 

Your current tire diameter is something close to 31.9", similar to my original tires when our truck had 3.73 axle gears.  A tire of 34" diameter, if it will fit your wheels and clear at the body's wheel wells, would make a considerable difference and effectively change your gearing.  Here is some quick math:  

1) 34" diameter tires with your 4.10 gears and 26% or 0.74 6th gear ratio would yield 1949 rpm at 65 mph.  

2) 33" diameter tires would yield 2008 rpm at 65 mph. 

3) For comparison, if your current tires are 31.9" diameter and you were to change to 3.73 axle gears, the rpm at 65 mph would be 1890 rpm in 6th gear overdrive.*

     *My 2005 5.9L truck's final drive axle ratio with the current 36.5" diameter tires is mathematically equivalent to 3.95:1 (hypothetical on paper, no such gear set exists) with its original 31.9" diameter factory tires. Your 6.7L ISB would easily pull 3.73s if 17K pounds is your max combination weight.  A good friend has a 2014 Ram Tradesman 6.7L 2500 4x4 with factory 3.42 axle gearing and the G56 manual transmission.  He's running oversized tires on top of the tall gearing and has pulled a travel trailer of 8000-plus pounds at higher interstate speeds.

If you have a tire store you deal with on a regular basis and they know medium duty truck tires and truck fit, see what's available in oversized tires that will fit your truck's chassis/body.  Make sure rim width is correct for the tire choice...Share what you find, and we can discuss this further.

When you change tire diameter, you will need to reprogram your speedometer for the new tire diameter.  This can be done without a large hassle, it's a software function and takes only minutes at the dealership once you know the exact diameter or revs per mile of the new tires.  I do this function within my Hypertech Max Energy tuning software.  Other software tuners and better scan tools have this function.

Moses

[w ranch]

Thanks  The truck is 4wd  DRW  have not weighed truck and trailer yet but the max on the trailer 17k will check the tag on truck can't remember what the weight is. I do pull hay trailer approx. weight 23k, around  10 miles but I don't want to limit myself with the truck. Sounds like 373 gears might be the way for me, will check on tires but with DWR I think I am stuck with the 19.5 I think the tire width will limit what is available. With G56 six speed and 373 gears would that combo pull these two trailer weights around here with steep hills and out on the highway. Were we live in the hills the terrain can be steep with no running room you can go around sharp curve say in 2 gear then straight up steep hills. I guess they don't make a manual transmission with like an over drive 6 where you could keep the 4.10 gears and then when needed on highway put in od 6 and drop the rpm some.

 

[Moses Ludel]

w ranch...According to Cummins, an ISB commercial engine should run between 2100-2400 rpm at cruise.  This is a medium-duty truck standard for maximum efficiency with loads to 50K pounds.  If your trailer alone weighs 17K pounds, I'm betting the combined truck/trailer over-the-scales weight is 25K-26K pounds.  That's considerable and has me concerned about how much fuel efficiency you'll gain from the change to 3.73 gears.  Here's the official Cummins standard for an ISB engine and an "Economy" fuel use goal:

1. Select a gearing combination that will result in an engine speed of between 2100-2400 rpm at the vehicle's intended cruise speed (mph).

2. For maximum fuel economy or for vehicles operating 65 mph or faster, select a gearing combination that will result in an engine speed of approximately 2150 rpm at 65 mph checkpoint.

3. Gearing combinations that produce an engine speed less than 1900 rpm at the vehicle's intended cruise speed (mph) should be avoided.

4. Note : ISB is not intended for weight over 50 000 lbs.

Note that the Cummins rule is 2150 rpm at 65 mph, slightly faster than your current speed.  Also, see the note about running below 1900 rpm.  Both of these sanctions relate to loaded medium duty commercial use trucks to 50K pounds gross weight.  (Visualize a box van loaded to the gills.)  If I run our non-catalyst 5.9L Cummins diesel engine at that speed, the fuel mileage drops dramatically every time I tip into the throttle and inch past 2000 rpm.  In the fall of 2015, on a trip running empty (maybe 8900 pounds GVW with fuel and light luggage) from Southern California home to the Reno Area, I tipped into the throttle and cruised from 2100 to a peak (briefly, approximately 100 miles) of 2300-2400 rpm.  Cummins is fine with this, according to commercial standards.  There were no concerns about damaging the engine.

Our truck has considerable wind resistance with a 4" chassis lift and big tires, a Warn winch bumper and the air dam removed.  The net result from these engine speeds was terrible fuel efficiency (around 10 mpg) with no trailer in tow and 35" tires.  The engine also developed a suspicious water pump seal and worn bearing/bushing leak, noticed 24 hours after spinning the engine at that speed for hundreds of miles.  (Coincidence?)  Replaced the pump and idler/belt tensioner at the same time, the only repairs I've made to this engine in 156,000 miles.  No more 2300-2400 rpm for longer periods, this is not a higher speed diesel by design and that applies to both the 5.9L and 6.7L ISB.

Your 6.7L Cummins ISB will easily pull the truck and trailer, even with 3.73 gears.  You do have 6 speeds in the G56, so there will always be a "correct gear" to take the load off the engine.  My experience says that running 1600-1900 rpm (around the torque peak) helps fuel efficiency tremendously, and when pulling a trailer, I'm very satisfied with a range of 1883 rpm @ 65 mph to 1999 rpm @ 69 mph.  

The caveat in your case is that 26K pounds is a significant load (not in terms of wearing the truck out) when it comes to getting fuel efficiency.  You may not notice a significant fuel efficiency gain with the change to 3.73s when loaded.  However, you would definitely know the difference when running empty (assuming 8500 pounds curb weight or so) or with a lighter trailer load.  Our trailering is all under 10K pounds trailer weight, typically a peak trailer weight of 8,400 pounds with a loaded 30' travel trailer.  That's around 17,300 pounds Gross Combination Weight (loaded truck and trailer).

Tough call, but if you want to get the rpm under 2000 at 65 mph in 6th gear, the 3.73s are a sure bet to achieve that goal.  I discovered how limited your tire choices would be with the 19.5" rims, and unless there is a sensible aftermarket wheel option that allows for 34" diameter tires, you have only a slight gain from the available 19.5" rim tires. If this takes us back to oversized tires, you need to confirm body and steering clearance for the new tires/wheels. 

Summed up, if you intend to always drive the truck with a 17K pound trailer in tow, the 4.10 factory gears are not a liability.  If you do drive empty or with a lighter trailer load at times, 3.73 gears would definitely make a difference in fuel efficiency.

Open for further discussion...Let us know what you decide to do here and the results.  Very interested.

Moses

[w ranch]

I was just looking for a way to get the RPM down a little not really looking for increased fuel mileage all though if I made any changes and that was part of result that is ok with me. A friend has almost the same truck his is 3500 with the automatic not sure which one, he also pulls trailer of same weight at 70 mph he is around 1800 RPM so that is why I was looking for a way lower RPM a little looks like it more trouble than it is worth. I guess his RPM is lower with the overdrive and I did not know if there was a gear that could be put in the G56 manual that would do that . I will be putting a clutch in soon and will have the trans out .

[Moses Ludel]

w ranch...Umm...Interesting idea about gear ratios in the G56.  You actually have the taller overdrive in the later G56.  Your overdrive gear is nearly the same ratio as an NV5600 6-speed that preceded the G56 in Ram trucks.  The G56 has a good reputation and is Mercedes built for medium duty trucks.  I'd stick with the G56.  Sounds wise to stick with the 4.10 axle gears, too.  That way you still have a true 4500 rated truck and not a truck that might struggle with maximum GVWR loads.  4.10s do take a load off your engine and transmission.

Glad we could air this issue.  The 4500 is a notch up to medium-duty level, and the gearing you have is the tallest that Chrysler will go with the G56 manual, consistent with the transmission's torque rating and load capacity.  A Ram 2500 with the 6.7L diesel and G56, by contrast, can come with 3.42 gears, but its load limit or GVWR is considerably less than your 4500.  

The consolation is that you have 4.10 gears instead of 4.44s, and you're also slightly below the Cummins recommended engine speed at 65 mph.  Cummins would have your engine at 2150 rpm, you're at 2100.  Chrysler must be following the Cummins guidelines when assigning gear ratios for your medium duty truck.

The engine can tolerate the speed, it's easier on the G56 with the 4.10s and a horse trailer.  As a point of interest, the weakest gears in the transmission are the over-driving gears, and these also have the heaviest load.  The more GVWR, the better sense to stick with lower (numerically higher) gearing.  4.10 is a good compromise for both load and engine speed/fuel efficiency.

Keep us posted...

Moses