Hey guys. Basically im about to own a 12v cummins converted truck. It has approx 390 hp and 900 or so ft lbs of torque and its mated to a nv4500. right now its rolling on 40's with a 4.56 gear ratio. im likely going to be going down to 37's. my question is, do i change the gear ratio to 3.73's or 4.10's?? i have looked at some of old gear ratio threads i found on here, but i didn't see anything that really specifically answered my question. im basically looking for a happy mixture of city and highway performance. also, if there's another forum this question would be better suited for, let me know. pretty new to the site. thanks in advance

 

i've got 4.10's and 35" tires and i like it. u should have plenty of power on the bottom and still be able to break 100mph with 37's n 4.10's

 

4.10's or 4.56's i would DEF change the tires first drive it for awhile then make your decision. if you pull much 4.56

 

I will be doing little to no pulling. Just want the best ratio for all purpose daily driving. I want the get up and go in the city, but I don't want 2500 rpm's at 70 mph either. 4.10's is sounding like the they may be the happy medium I'm looking for. Thanks for yalls input.

 

My old man drives a similarly powered 12 valve on 37's with 3.54's with great results. It takes off effortlessly in 2nd and has had loads of over 25k and never has to come out of 5th while hitting hills on the interstate. It runs around 1,550 rpm at 65 and will get 22mpg on highway only runs.

 

interesting. since this truck was a conversion project, it actually doesn't have dodge axles. its got ford axles and the guy selling it to me does have an extra set of 3.73's he's going to give to me. if your dad's runs that great with 3.54's then i may give those 3.73's a try since ill already have them. hard to argue with 20+ mpg's.

 

I'm running 38"s and 4.10's and wish I had higher gears because 1st gear is worthless and I'm turning about 2k at 75mph in 5th...ultimately resulting in 16-18mpg considering the ol Fummins has the aerodynamics of a 4x8 sheet of plywood turned broadside. I'd say that, considering you aren't going to pull anything, the 3.73's with 37"s should be a pretty good combo.

 

thanks man. since ill already have the 3.73's, i think im going to go ahead and give them a shot first. if i feel like im wanting more, i can always move up to 4.10's later.

 

Tire Gear Ratio Charts
there ya go

 

If you had factory 4.10s id leave them. But if you had 3.54s, Id swap them to a 4.56 ratio