Engine/Transmission (1998.5 - 2002) The right gear ratio for 315/75/16s?

[Nate]

Right now I'm running 3. 55s with 315/75/16s.



It's fine with unloaded driving, but when I'm pulling my Jeep loaded on the car hauler (6500-7000lbs), I find that it's quite sluggish... that I'm having to work the engine more than I'd like.



The torque convertor could be part of the problem (stock rebuilt one). I'm curious what the thought is on the proper gear ratio for 35" tires?

 

[EBung]

4. 10s would put you close to what you had with 3. 54s and stock tires. go to www.reiderracing.com to do the math. Go to reider tech and calculate gear ratios. Assuming a stock tire height of 32" a 3. 73 would put you about 70 rpm less with 35s than you did stock.

 

[Hohn]

With an auto trans, I'd go with a 4. 56 or thereabouts for towing. That would put you around where a 4. 10 is with stock tires.



With a 6-speed, I'd go with 4. 10s on 35s.

 

[jraker]

Nate,

I am running 4. 10's now with 315/75r16 BFG AT's. I have not run into an issue pulling and the truck being sluggish. I have a full DTT trans but even before that my truck would accelerate under load up grades in Northern Idaho and Washington State. I think 4. 10's would be the ticket.

 

[Nate]

What is considered the ideal RPM "range" for the Cummins engine for power and economy doing 65-70mph?



Right now it's about 1800rpm.

 

[RJPotts]

I'd say at 1800 you're about there. I think peak volumetric efficiency (torque) is somewhere close to 2000 rpm on these engines. And 1800 seems to be about the best for fuel economy. Considering you can keep it there.

-R. J.

 

[LOtero]

when i went from stock tires to my 315's i felt the same power loss when towing. the programmer helps a lot, I tried towing my scout with out it and didnt like it much. with the superchip or ez and intake exhaust it should help a lot. this weekend i pulled my buddies 78 4x4 pickup, 85, 90 once down the freeway with no prob. on not so steep hills i kept 80.

 

[BigEasy]

Jeez... I have 3. 55's and I think they'll be just right when I go to 35's... remember, you can always gear down... you run out on the top end.

 

[Nate]

I'm not worried about the top end, as the truck doesn't see anything over highway speeds.



I have a SuperChip programmer I got with the truck, though I undid the programming when I pulled the trailer since I didn't have any gauges. Haven't put it back on... don't really like it that much.

 

[SKneeland]

my 3. 55s work just fine for towing

 

[Nate]

Yeah that 6 speed helps too.



I drove out to Wasilla last night with the truck and 65mph is actually 1600rpm, not 1800 that I thought it was. I'm running about 800 for EGT and 5-7psi boost at that.



It's not too bad unloaded, but when pulling a trailer, more so going up grades, I find I have to work the engine harder than I'd like. I bet the transmission loves it too.



I'm not sure the route I should take, upgrade the transmission (think DTT torque convertor at least) and maybe some larger injectors... or keep the engine stock and regear. I think either way I'm going to do some transmission work though.

 

[MattSt]

A comp box with an untapped wire (save the trans) will make it feel faster SOP meter than stock with 265's. That's what I did and love it. Gets better mpg than the 265's also.

 

[Nate]

I have a Superchip programmer on it.



It's not so much that the truck feels like it's lacking power, just that it feels like I'm "lugging" the engine with the RPMs being so low.

 

[Nate]

Well I did some calculating... even had to take my shoes off :-laf



Stock truck with 3. 55s @ 65 ~ 1800 rpms

Stock truck with 4. 10s @ 65 ~ 2050 rpms

My truck with 3. 55s @ 65... ..... 1600 rpms



4. 10s would put me at about 1850 rpms

4. 56s... ... ... ... ... ... ... ... ... ... . 2050 rpms



What is considered to be a good cruising RPM for the Cummins? I'm guessing around 2000rpms?



Yes I can build up the engine... but that going to make the transmission work harder, so I'd rather have the proper gears.



With the added drag of the tires, I'm thinking 4. 56s would be better... . opinions?

 

[Bob Wagner]

1900 is the sweet spot

 

[Nate]

Would it make sense to go a bit under the sweet spot, or a bit over??



I did some more testing last night. My speedo was off a bit, and 65mph (with GPS) is actually ~1550 RPMs.

 

[MattSt]

Would it make sense to go a bit under the sweet spot, or a bit over??



I did some more testing last night. My speedo was off a bit, and 65mph (with GPS) is actually ~1550 RPMs.

You planning on going faster to hit the sweet spot? Or slower?



You get on the interstate here in NE and go under 80 and you get run off the road. So keep in mind that alot of interstate roads have a speed limit of 70-75.

 

[Nate]

I bought this truck to pull trailers, not to win races, but I know what your saying. I'm more worried about low end power though. At long as I can do 65-70 mph without getting carried away with the RPMs, I'm fine.



I am taking it 4. 10s and stock tires is "the suck"? So 4. 56s and 35s would also be "the suck"?

 

[nps]

I've got 35" tires along with a 6 speed and a 3. 55 rear end.



With the stock 265s, even 2nd gear was too low for empty launches. With the 315s 2nd gear is just right. When towing my 10k trailer, first gear pulls just fine from a stop.



I think you would benefit more from a good tq converter and transmission than gears.

 

[Nate]

The thing is though, I'll still be running at 1550rpms doing 65, which almost lugging the engine IMO.