I just bought a 2002 dodge 2500 4X4. It came with a 5sp and 4:10 gears. It turns about 2100 rpm's at 70 mph. It has 315-75-R16 BF Goodrich A/T tires. I bought a set of gears front and back for the truck. They came out of a truck that was the same year and had the same options. I was told that they were 3:55's. How can I tell what they actually are? What are your opinions on changing them?

 

3.55:1 39-11
This is for another axle, but the ratios should be the same. You divide the small number (count the teeth on the pinion) into the big number (count the teeth on the ring gear). You wind up with the actual reduction in the rear end.
It could be 46-13, which rounds down to 3.54:1
Anyway, cound your teeth. Divide small number into large.
With 315 tires, it almost makes up for the too-short 4.11's. Your rpm is about what mine is with 3.55's and 285-75R16 All terrains. I turn 1600 rpm @ 55 in 6th, and around 1975 rpm @ 70mph. 1900-2100 is a sweet spot with these 2nd gens.
I wouldn't change the gearing, as I think it will lug down too slow and you won't be using 5th gear as much. Only if you went back to smaller tires and were not hauling heavy loads or towing big stuff.
I'm not sure what you're trying to achieve here.
regards, as always, jefe

 

I will very seldom tow anything over 5k pounds. I just thought I might get better mileage with the higher gear ratio. I'm not sure about the mileage now. I havent even ran a complete tank of diesel yet. The gears only cost me fifty bucks. I'm not real concerned if I don't use them.

 

I beleave some 2500's had dana 80's ans some had 70's in them depending on the tranny . might wont to check this out before you get started. It should help you milage 2-3 mpg I would think

 

the manuals had the 80 and the automatics had the 70

 

I have the 6spd with the 4:10's, and I will be putting 315's on the rims very shortly. I would not swap out the gear ratio in my truck. Yeah, it is geared a little lower, but my truck pulls VERY hard in every gear, and I LOVE it. LOL Btw, I get about 19-22 MPG in my truck which is good enough for me!

 

You might want to veryify your speedo... If you are running larger tires on an uncorrected speedometer you will calculate a lower MPG than you are actually getting, also, you may be going faster than 70 mph @ 2100 rpm if uncorrected. I have the 3:55 gears and wish I had the 410's with stock tires. When towing heavy I end up running between 70 and 75 to keep rpms at around 2k.

DuaneW.

 

If you are only wanting to swap the gears for improved fuel economy, I would not do it. I would sell the gears on here in the classifieds since the 3.55/4.10 swap is pretty common, with people wanting to go both ways. Anyway, I had 4.10's and made the change out to 3.55's with big hopes of a decent improvement in fuel economy..........with almost no change. If I really baby it I can get 1mpg better with the 3.55's, but I never really babied it with the 4.10's, so I'm not sure I couldn't have gotten the same mileage with either. I do think the 4.10's with 315 tires is a good combo.........smaller tires would complement the 3.55 gears better, and will accelerate quicker in my experience.

Waylan

 

You are correct about that. I have taken that into consideration and I figure that the difference is about 11% in my favor. My 255/85's have an increase in diameter of 11% over the tires that came stock with the truck. Therefore, my mileage is actually 11% better than what my overhead is telling me which is the corrected number that I posted earlier in this thread. But thanks, I'm sure there are alot of people out there who don't realize how much of a difference a slightly taller (or smaller) tire can have on your speedometer/odometer readings.

 

Even with your correction factor, I doubt your overhead is accurate. Only way to be accurate is to hand calculate.

 

I did, my overhead hits my 11% variance on the nut, when I compare my Odometer to gallons of gas used (at the pump) and then add 11% to that.

 

I swapped from 3.55's to 4.10's in a six speed for towing and I run 315's and I like the swap. I really only use the truck for towing. Running at 70 I am at 2100 where before with the 3.55's I was running almost 80 at 2000 rpms. Fuel economy went down about 2 mpg. I am half way to saving up for a us gear splitter. Best of both worlds. Then my truck is finally finished. I HOPE.

 

Thanks for all the information. I't sounds like I should leave the truck alone. Does anyone know about how much 3:55 gears go for? I have front and back for a 2002 dodge.