I am starting on a new jeep build, and am in the process of building the axles (Front 60 and shaved 14bolt rear). I am planning on running an auto (probably 700r4 but maybe th350 or even 400). 40" tires are also likely. HP is unknown but will be a lot. I have never wheeled an automatic tranny and have no idea what final gear ratio to shoot for. I need to order gears right away and need to decide between 5.38s and 5.13s. I will buy an Atlas which gives me even more gearing options to think about so any input on that will be welcome as well. I will never be doing 70 or 80mph but may want to occasionally drive 55. What do you guys with a similar set up run in your diffs and how do you like them.
Ring and pinion ratio with 40s and automatic.
- Thread starter bobdog
- Start date
TEAM FRED
Registered User
- Location
- Centerville
Depends what lots of horsepower is. But I would say 5:38s
It also depends on whether you have an overdrive trans or not. But, there really isn't too much difference between 5.13 and 5.38. So get whichever is cheaper. 
My old buggy had decent horsepower (not "lots", but probably 350) and I had 5.13's in the diff with 37's and then 40's. TH400 trans. I was very happy with it, with my 3.8 Atlas. It crawled just fine, and would still do 100 in the desert. (I'm told...I never had it anywhere near that)
My old buggy had decent horsepower (not "lots", but probably 350) and I had 5.13's in the diff with 37's and then 40's. TH400 trans. I was very happy with it, with my 3.8 Atlas. It crawled just fine, and would still do 100 in the desert. (I'm told...I never had it anywhere near that)
Jinx
when in doubt, upgrade!
- Location
- So Jordan, Utah
Ok I am going to go against the grain. I run a 700r4 with 4:56s and 40's and it seems to go pretty well.
I do run a 4spd atlas with 2.72 and 3.8. On the trails I usually run in 2.7 and hard obstacles 3.8.
The difference between the 700r4 (3.06 1st gear) and the TH350(2.52) is probably why I can get away with it.
(The final drive ratios will be close if your running 5.13s with the TH350)
In my head I like the thicker teeth but probably it doesn't really matter.
I do run a 4spd atlas with 2.72 and 3.8. On the trails I usually run in 2.7 and hard obstacles 3.8.
The difference between the 700r4 (3.06 1st gear) and the TH350(2.52) is probably why I can get away with it.
(The final drive ratios will be close if your running 5.13s with the TH350)
In my head I like the thicker teeth but probably it doesn't really matter.
keep i mind that a 40" is 30% taller than a 28.5" tall tire. Which is what most factory jeep tire sizes are (for example an XJ).
5.38 is essentially 30% shorter than 3.76.
so....
At 5.38, the rig will roughly perform like a rig with stock gears, and stock tires with the same powertrain. So basically it would have the same final drive ratio as, and would act like a stock chevy truck in 2-hi (since that's the powertrain you are aiming for).
5.38 is essentially 30% shorter than 3.76.
so....
At 5.38, the rig will roughly perform like a rig with stock gears, and stock tires with the same powertrain. So basically it would have the same final drive ratio as, and would act like a stock chevy truck in 2-hi (since that's the powertrain you are aiming for).
Last edited:
Hard for me to say much due to my tranny and not going out a lot. But I have a th350, 203/205 doubler, 40" tires. I run 5.38's and I like it. It's a strictly off road road. Like Carl said though, I doubt there is much difference between the two.
ufab- east coast gear supply had a 5.38 for a hp super
ufab- east coast gear supply had a 5.38 for a hp super
4554x4
always modifing something
- Location
- Sandy Utah
I run a stock TBI 350, turbo 400, 203/205 doubler, 5.13 gears and 38.5 tires. I feel that with all my additional weight, ( winch, cage, one ton axles etc.) that it performs like stock on the streets. In double low it has pretty good grunt. Next move, 6.0 LS and 4l80e! Won't be soon though.