Ram 2500 XL for overland / expedition trips

[chet6.7]

This is a C&P from the CF by the member TransEngineer about the effects of larger tires,added torque to the trans,and gear change.

"Actually, changing the tire size WILL affect the trans life. It's effectively like changing to an even lower (numerical) axle ratio. This increases the reflected inertia of the truck (basically, the amount of torque required to accelerate the truck). So you'll be operating at lower engine speeds, but at higher torque, than you would with the stock tires.
Transmission life goes down as torque goes up. And the relationship is not linear. For the needle bearings in the planetary gearsets, the life goes down by a factor of torque to the 10/3 power. Not only that, but the reflected inertia goes up by a factor of tire size squared.
So, for example, if your tire size (rolling diameter) increases from 31.07" (normal size for a diesel truck) to 35", then your reflected inertia will increase by 26.9%. That then translates to a bearing life decrease of almost 55%. So with 35" tires, I'd expect your transmission life to go DOWN by as much as 55%. Good luck with that. Don't expect your dealer to cover a trans failure under warranty if you use larger-than-stock tires."

His second post in that thread,titled "Will the trans mess up?"
"If you change the axle ratio, to compensate for the increased tire size (getting you back to the same overall ratio, as you noted), then you'll be OK (no detrimental impact on trans life)."

 

[marshal]

This is a C&P from the CF by the member TransEngineer about the effects of larger tires,added torque to the trans,and gear change.

"Actually, changing the tire size WILL affect the trans life. It's effectively like changing to an even lower (numerical) axle ratio. This increases the reflected inertia of the truck (basically, the amount of torque required to accelerate the truck). So you'll be operating at lower engine speeds, but at higher torque, than you would with the stock tires.
Transmission life goes down as torque goes up. And the relationship is not linear. For the needle bearings in the planetary gearsets, the life goes down by a factor of torque to the 10/3 power. Not only that, but the reflected inertia goes up by a factor of tire size squared.
So, for example, if your tire size (rolling diameter) increases from 31.07" (normal size for a diesel truck) to 35", then your reflected inertia will increase by 26.9%. That then translates to a bearing life decrease of almost 55%. So with 35" tires, I'd expect your transmission life to go DOWN by as much as 55%. Good luck with that. Don't expect your dealer to cover a trans failure under warranty if you use larger-than-stock tires."

His second post in that thread,titled "Will the trans mess up?"
"If you change the axle ratio, to compensate for the increased tire size (getting you back to the same overall ratio, as you noted), then you'll be OK (no detrimental impact on trans life)."

that math makes absolutely no sense. the truck was built for hauling on a commercial spec. A minor increase in rotating mass is SIGNIFICANTLY more detrimental to the life of the transmission than a say a 10,000lb trailer behind it its entire life? Not to mention the vehicle is optioned with 4 different gear sets from the factory. I call ****************

Obviously its going to affect its life, but going from a 31 to a 35" tire is going to shorten the life of the transmission by upwards of 50%? laughable.

 

[chet6.7]

TransEngineer has a lot of knowledge about the 68RE trans,I believe he worked for Chrysler on that trans. A review of his posts on CF will demonstrate his knowledge of the trans.
As always, folks are free to make their own decisions in life.

 

[Bayou Boy]

that math makes absolutely no sense. the truck was built for hauling on a commercial spec. A minor increase in rotating mass is SIGNIFICANTLY more detrimental to the life of the transmission than a say a 10,000lb trailer behind it its entire life? Not to mention the vehicle is optioned with 4 different gear sets from the factory. I call ****************

Obviously its going to affect its life, but going from a 31 to a 35" tire is going to shorten the life of the transmission by upwards of 50%? laughable.

The biggest problem with that math is that a lot of Rams come with 33.5" tires stock. Moving to a 34.5" tall "35" is a negligible change. I noticed almost no difference going from my stock 285/60r20s to 35" tall 295/65r20s. And it towed my 12000# boat just fine with the stock gears. With my current 36x13s, not so much. I'm looking to go to 4.10s in the summer.

 

[ttengineer]

In my 3500 I've found, unloaded, my best fuel economy at 70mph is in 5th gear at 2000rpm getting 21mpg average.


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Which transmission do you have and how many gears?

68RFE

It’s a 6 speed automatic. On highway trips where I travel more than 50 miles with no congestion I always drop the trans into 5th. It kicks the RPMs up a little, but diesels run more efficiently at a slightly higher RPM. The sweet spot is right at 2000RPMs ... for efficiency.


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[marshal]

68RFE

It’s a 6 speed automatic. On highway trips where I travel more than 50 miles with no congestion I always drop the trans into 5th. It kicks the RPMs up a little, but diesels run more efficiently at a slightly higher RPM. The sweet spot is right at 2000RPMs ... for efficiency.


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curious to know what your thoughts are on RPM efficiency for the 6.4

 

[Ravenmad]

considering tires, I bought Continental MPT 81s. I did look at a lot of other 40s, all C and D rated side walls. Super swamper was not an option for daily driving as they won't last very long. I spoke in length with DPP and for them, they run Toyo's in the 40 inch variety with 17 inch wheels. For daily driving they are very good, but for towing the tread life fades REALLY fast, same with most other 40s for 17 inch wheels. I want to be able to pull a 18K trailer and not watch my tires drop 4-5/32 on a trip to Moab and back.

I totally get that the tires I bought are only speed rated for 68mph under max load of 6900lb per tire. However, this is also in part why I am gearing the truck to run at optimal RPM at 70 mph (1850), if I downshift for mountain passes to 5th gear, no big deal, puts me at about 2300 rpm ( about max torque and HP ). Im just not the kind of driver that cruises on the highway at 80-90 so no big deal to me.

I understand the added stress on the trans. and drivetrain parts by running a larger tire without a gear swap to match. Years of building jeeps has taught me any time I can eliminate stress on parts the better. I don't think going up 1-2 inches in tire size is that detrimental ( I actually think the trucks are under geared to start with as I think they are largely built for the casual consumer who just wants to drive their truck and maybe pull a small trailer every now and then). If your towing, just flip the tow haul mode and run in 5th gear I suppose. Just my thoughts, but then again, Im installing 4.88 gears and running a 41 inch tire.

 

[DT75FLH]

So could you expand on this bit? I've got a friend looking at bigger tires (35's) for his Ram w/ Aisin transmission. Stock gearing (for SRW) is 3.42. We've been debating back and forth on this. Some owners claim that 3.73's should be low enough to get the RPM back into OEM-like specs. Some even argue that stock gearing would work (I'm highly skeptical of those claims). Others say 4.10's or lower are needed.

What is AEV's mindset on gearing? Do they recommend going with gears that are low enough to get the RPM's back into OEM territory for highway cruising, or do they recommend going lower for accommodating increased weight and reduced aerodynamics?

I have towed 16k on a ram 3500 with 34.8 in tire. In Utah some of the speed limts are 80. If the speed is 75 and your worried about the gear ratio you can lock out 6th gear and be In Single od, and have the same gearing as 4.10 with 35 in tires. My stock 20 in tire was 33.5..new tires were 34.8 (285-75-18).

With 900 lb ft the new trucks can tow with the 3.42. And the aisin trans has a deeper 1st gear vs the 68rfe.

 

[DT75FLH]

I have owned numerous jeeps and towing trucks...All te jeeps got deeper gears when lifted, and my 2002 f350 with 4in and 35 in tires I had 4.10 installed and did the same with my 2008 f350...my last 2 rams that had 650 lb ft and the 900 lb ft did not get new gears. I was surprised by the power with 3.42. My Intial plan on paper was to swap gears with the larger tires...till I towed a few trips with them. I live out west also and routinely tow in the mountins, up some 6 to 10 per cent grades. If the trans start to hunt I lock out the gear...I usually drop a gear at the base of a hill before the cruise does it. Towing in 120 deg heat with no temp issues etc

 

[Dalko43]

I have towed 16k on a ram 3500 with 34.8 in tire. In Utah some of the speed limts are 80. If the speed is 75 and your worried about the gear ratio you can lock out 6th gear and be In Single od, and have the same gearing as 4.10 with 35 in tires. My stock 20 in tire was 33.5..new tires were 34.8 (285-75-18).

With 900 lb ft the new trucks can tow with the 3.42. And the aisin trans has a deeper 1st gear vs the 68rfe.

Yeah, I agree the torque and power delivery on these newer engines is a bit different from the prior diesels.

I'm still a little confused on what RPM's are ideal for the 6.7l Cummins for both efficiency and work.

As it comes from the factory (SRW w/ 33" tires and 3.42's) the engine is turning 1430 RPM's @ 65 mph and only about 100 RPM more @ 70mph, both of which seem extremely low. But that's how FCA sets up and warranties the powertrain, so I have to believe that the engine is not only efficient but also safe at such RPM's.

But I've heard more than a few owners claim that ~1700 RPM is the efficiency sweet spot and that for any sustained work (towing) RPM's should be kept above 1900 RPM's (supposedly there was a bulletin from Cummins which specified that work RPM).

So the question is: if you're re-gearing to accommodate bigger tires and modifications (lift, armor, auxiliary fuel, ect.), should you choose a gear set which puts you back into the OEM RPM ranges or one that puts you into slightly higher RPM's?

 

[Ravenmad]

$1,000,000. question....... I think its preference and opinion.... Everyone has an opinion and a preference, I personally have to defer to what it is "you" want and then just trust that its your preference and money... My truck will be set to operate at about 300rpm above the factory / stock rpm at 70 mph (i.e 33s with 3.42 gears stock, 41s with 4.88 gears when done). You are correct, stock my truck is almost idling down the road at 55mph. just my thoughts

 

[chet6.7]

Cummins provides this,I have not used it.
https://cumminsengines.com/powerspec

Everything to know about your engine.
Cummins PowerSpec is an electronic tool that makes it possible to tailor the operation of Cummins engines in every on-highway engine application. PowerSpec is compatible with Windows 7, Windows 8 and Windows 10.
PowerSpec 32-bit download button
[alternate download location]

PowerSpec 64-bit download button
[alternate download location]

Need help with PowerSpec? Call 1-800-CUMMINS and select option 2.
Looking for specific Engine Information?
If you’re looking for particular information about an engine, like fan control or accelerators, you can now find that information under Engine Feature Descriptions.
About PowerSpec
PowerSpec's full functionality is available to support all Cummins 2013 On-Highway engine products (ISX15, ISX12, ISL9, and ISB6.7), 2010 engine products (ISX15, ISX12, ISL9, ISC8.3 and ISB6.7), 2007 products (ISX, ISM, ISL, ISC, ISB), some of the Natural Gas products (ISL G and ISX12 G) and earlier products. Features include a gearing calculator, complete descriptions of electronic features, instructions on how to optimize parameter settings, and access to trip information and fault code data. It also supports fault codes and trip information on Euro 3/4.5/5, ISBe, N14+ and M11+ CELECT Plus engines. Other functionality is not available for the Euro 3/4.5/5, ISBe and Celect Plus engines.
PowerSpec can be downloaded, at no charge, to any PC running Windows 7, Windows 8, or Windows 10. You will need the INLINE 6 adapter cable, available for order at the Cummins PowerStore or through your Cummins distributor, to communicate with and gear your vehicle, configure electronic features, and read engine data.

Please note: The use of PowerSpec is supported in North America only. For engines equipped with a Diesel Particulate Filter, Ultra Low Sulfur Diesel Fuel is required. Failure to do so can permanently damage engine and aftertreatment systems within a short period of time. This damage could cause the engine to become inoperable and affect the warranty coverage on the engine.

Click here to download the PowerSpec 6.0 user manual.

For help with PowerSpec, call 1-800-CUMMINS (1-800-286-6467).

 

[Explorerinil]

I have towed 16k on a ram 3500 with 34.8 in tire. In Utah some of the speed limts are 80. If the speed is 75 and your worried about the gear ratio you can lock out 6th gear and be In Single od, and have the same gearing as 4.10 with 35 in tires. My stock 20 in tire was 33.5..new tires were 34.8 (285-75-18).

With 900 lb ft the new trucks can tow with the 3.42. And the aisin trans has a deeper 1st gear vs the 68rfe.

Yes your right they will tow and do it well, the low first gear of the Aisin will help you get to a good start... however with 3:42’s and larger tires your rims will run out of its power band and the trans will downshift allot more often, I ran 3:42 gears and 35’s and 37’s... you will notice a difference with bigger tires.

 

[ttengineer]

68RFE

It’s a 6 speed automatic. On highway trips where I travel more than 50 miles with no congestion I always drop the trans into 5th. It kicks the RPMs up a little, but diesels run more efficiently at a slightly higher RPM. The sweet spot is right at 2000RPMs ... for efficiency.


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curious to know what your thoughts are on RPM efficiency for the 6.4

Gas motors run best at around 1300-1500 rpm. I've never run a PW before so I have no experience to pull from, but typically the range above should provide you the best results in an overdrive gear.


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[Dalko43]

@chet6.7, I downloaded the most recent version of Cummins' Powerspec program.

The problem is that it is geared (no pun intended) towards true medium and heavy duty applications of the 6.7l Cummins (RV's, buses, small transport trucks, ect.). The RPM's that it recommends for for the 6.7L (1700-1900 RPM's for economy and 1850-2150 RPM's for balanced performance) tend to be higher than what the stock Ram 2500/3500 turns with 3.42's and stock tires.

It seems like there is a discrepancy between how FCA sets up and gears the 6.7L Cummins in their trucks and how Cummins suggests its engine should be geared.