Transmission swap

[Halfdane]

2016 ram 3500 6.7 Cummins 4x4 turbo diesel white 9ft bed crew cab, ~275,000 miles

I have a question about doing a transmission swap for this truck. The transmission on it now is the higher ratio one, the 4.38:1, or whatever the correct number is, I am admittedly ignorant of the right number. The transmission has no mechanical issues.

The reason for the question is that the truck leaves a lot of power by the time I hit sixth gear. I’m running 2,000rpm at about 75-80mph, whether or not I’m using tow/haul mode. This is a “workhorse” truck and it pulls trailers with small tractors and occasionally an RV. More often than not it’s an everyday truck for my business. When I’m on a highway and the speed limit is 75mph, I’m wanting to keep a lower steady RPM for better gas mileage.

What are the best options? I want to make the truck last, so even an expensive transmission is better than a whole new truck right now. I’ve heard there are transmissions with more gears. Any guidance would be greatly appreciated. TIA.

 

[Brandon-w]

Gear swap or bigger tires are your best options. The cab and chassis trucks don't have as high of tow capacity as you'd think. They actually have a heavier payload than non cab and chassis trucks 2024 rates the 3500 at something like 11,800lbs the regular 3500 is good for 6830lbs I Beleive.
The c&c trucks only come in a 4:32 gearset or whatever it is. Which doesn't really help for towing capacity the 3:73 trucks can tow 2000lbs more than c&c trucks. I learned this building and pricing one I wanted, figured ooh 5500 cab and chassis = more capable but turned out not to be as such.

My old 95 I have 4:11 Gears and a 5 speed manual and she howls at highway speeds. I upped the tires from 29 to 33 and that dropped me down 100 rpm. I'm going to 35s soon and that should drop me to the equivalent of 3:73 ish Gears. Transmission swap won't give you any advantage in my opinion. You already have probly a .72 overdrive ratio mayve even lower which is pretty good. It's all tires and Gears.

 

[Halfdane]

Gear swap or bigger tires are your best options. The cab and chassis trucks don't have as high of tow capacity as you'd think. They actually have a heavier payload than non cab and chassis trucks 2024 rates the 3500 at something like 11,800lbs the regular 3500 is good for 6830lbs I Beleive.
The c&c trucks only come in a 4:32 gearset or whatever it is. Which doesn't really help for towing capacity the 3:73 trucks can tow 2000lbs more than c&c trucks. I learned this building and pricing one I wanted, figured ooh 5500 cab and chassis = more capable but turned out not to be as such.

My old 95 I have 4:11 Gears and a 5 speed manual and she howls at highway speeds. I upped the tires from 29 to 33 and that dropped me down 100 rpm. I'm going to 35s soon and that should drop me to the equivalent of 3:73 ish Gears. Transmission swap won't give you any advantage in my opinion. You already have probly a .72 overdrive ratio mayve even lower which is pretty good. It's all tires and Gears.

Thank you. About the tires, I have 235/80R17s on it. Not sure how equates to 29s or 33s. I am actually glad you recommend tires because this was another question I had. I replaced the tires recently and at that time considered going to 255/80R17 because they are less expensive and appear easier to source.

What are the risks of a bigger tire? The truck is a dually, so I’m being mindful of tire rub at any point. Does it affect the speed readout, or is that ECM calculated and will display correctly on the dashboard? Thanks again.

 

[Brandon-w]

255/85r17 would put you at 34.1 inches in diameter and 1inch wider so 1/2 inch wider per tire edge which shouldn't affect your dual clearance. This will require a speedo correction but if you have a tuner or use alfaobd (look into alfaobd) it's easy enough to fix this issue and make it true again. At 60 mph you'd be at 64.3 and at 80mph you'd be at 85.8 mph.

So assuming you have the 66rfe your 6 gear overdrive ratio is .62. Meaning at 70mph your rpm is roughly 1985rpm. If you go with 255/85r17 your rpm would be 1851 rpm that's 134rpm drop.

It doesnt sound like Much but that made a huge difference just going 29 to 33 on my dually and it'll be huge when I go to 35s.

Hope this helps

 

[Halfdane]

255/85r17 would put you at 34.1 inches in diameter and 1inch wider so 1/2 inch wider per tire edge which shouldn't affect your dual clearance. This will require a speedo correction but if you have a tuner or use alfaobd (look into alfaobd) it's easy enough to fix this issue and make it true again. At 60 mph you'd be at 64.3 and at 80mph you'd be at 85.8 mph.

So assuming you have the 66rfe your 6 gear overdrive ratio is .62. Meaning at 70mph your rpm is roughly 1985rpm. If you go with 255/85r17 your rpm would be 1851 rpm that's 134rpm drop.

It doesnt sound like Much but that made a huge difference just going 29 to 33 on my dually and it'll be huge when I go to 35s.

Hope this helps

Absolutely it’s been helpful, thank you. What are the 35s? Would that be 275/85r17? I do have a tuner so I will make the appropriate adjustments to the speedometer.

 

[Brandon-w]

I'm not sure if you'd clear 35s without modification to plastic on your front end. Minor plastic trimming would likely be required. You'd be better off going with a 35/11.5r17 the 275/85r17 is 35.4 inches tall. The 35s would probly be more available in different styles as well. You'd be at 1800 rpm at 70 mph with these tires. 185rpm drop.

 

[Halfdane]

Okay, thank you so much!