I've got an '03 2500, 4x4, 5-Speed, w/ 4:10 gears, and the S.O. Cummins (250 hp / 460 tq). I'm also running a SMARTY on the 60hp level.

I've heard that we now have an option of 3:42 gears and I'm thinking of swapping my 4:10's.

My towing at most is a 16' bass boat and an 8' flat bed trailer with two 4-wheelers. I do a lot of flat highway driving - no mountains in TX.

My RPM's at 70mph are about 2100-2200. I'm looking to improve my fuel mileage (average 18 mpg). And with the 3:42 gears, I'll probably be able to use 1st gear, since right now it's a granny gear.

With my tire size of 315/70/R17 and the S.O. Cummins, will the 3:42 be a bad choice? Will I have enough power to smoke the tires from a dead stop like I can now?

 

342's will drop you to just over 1600 @ 70, 1850 @ 80
it'll cost ya around a grand in parts + labor.....

 

Wow, just over 1600 rpm's at 70 mph??!! If i'm getting 18 mpg now.....just think what i'll get with 3.42's! That and i'm sure i'll be able to use my 1st gear! The more i read up on this, the more i'm thinking i'll do it.

 

If you do swap gears I may be interested in your 4.10's

 

I would say worst casse scenario you would use 6th gear less and maybe nest time you buy tires you drop down a size which is not a bad thing imho

1000 bucks front and rear I think I may do this as well

 

I don't have a 6th gear. It's a 5-speed Standard. That's why I was wondering if I've got enough engine to turn 315's with the 3:42 gears. I know the SMARTY will help, but it's only set on 60hp.

 

If you change to 3.42 your gear steps with that tire size will be wrong. I really doubt you would see any improvements because of the tire size. If you are in the 2100-2200 at 70 mph range, you are pretty good already. Do a search for the gear calculator and look at your steps for the gear if you change. Your truck may fall flat on its face at highway speeds if it fall off the torque band.

 

another way of looking at it is how many people ran a similar set up with the old 12 vlves when they were out with the 3.55s hp was less rpms were definately less mpg was up if you had the six speed I think it would be great but the 5 speed could leave you wanting a little more and if thats the case get a shorter tire

 

The one thing I don't want to lose is horse power! W/O the SMARTY, the thing is a dog. Now I'm a little concerned that 3.42's will be too high of a gear and I would lose the thing I want most - HP!
I like the 315's - they fit the truck nicely and i like the ground clearance they give me, so I really don't wanna go to a smaller tire. I guess I'm stuck where I'm at and should appreciate getting 18mpg out of a 6,000 lb 4x4 brick!

 

Split the difference with 3.73 and be happy!

 

3.73's with 315's is perfect. That's what I ran on my '03. Around 1800 at 70 IIRC.