I have an '09 JK, auto trans, with recently added 35s. I get out a few times a year for a little crawling, maybe more now with the group I just joined. Do I change my 3.73 stock gears to 4.10 to keep close to factory? Go with 4.56 to get a little more low end for the crawling and keep RPM's under control on Highway? Or the much loved 4.88 and just realize that I won't be passing anyone at 70mph?
I am strongly considering 4.56 and maybe tossing in an electric locker on the rear for now?

 

Depends on what ur using ur Jeep for. U got the stock 3.8 motor assuming so for Daily driving, moderate trails maybe 4.56 or 4.88. More extreme use go with 5.13 or 5.38!!

 

If it wasn't a daily driver, I would definitely go into the 5. range. I do have an eight hour trip coming up, Jeep Jamboree Palo Duro, for some wheeling. 4.56 might just be the route I take, so I am not long hauling at 3000 rpm's.

 

Press the OD/Off button and get up to 70 on the freeway. You're sitting right about 2570ish rpm? Right about where 5.38's would put you with OD/On. Fairly good way to make a seat of the pants comparison, anyway. (The chart in the FAQ's stuck to the top of this modified area is pretty good for the 3.8 auto. To get OD/Off, just divide the rpm it gives by .69)

Many may/will disagree, but I like being up around 2500+/- rpm on the freeway with my 3.8 auto jku. Gives enough HP/TQ to keep my brick moving. (I ran 5.13 with 33's, then 35's, then 5.38 with the 35's, and now 37's. Mixed time between Austin near sea level and Denver at altitude.) What is right for you will depend on your use/needs/location/opinion etc etc. Just think about it for a while before dropping the cash...

 

if you go from 3.73 to 4.10 you're an idiot

 

2570 with it off at 70? For 3.73? I'm fairly sure that I'm about 3k rpm at 70 with OD off. 4.10's and 35's. I could be wrong

 

Open the charts and do the math. It has been pretty spot on for me through 3 different ratios and 5 different tire sizes.

3.8 auto, 35's (that measure 34" ??), 4.10's, and assuming a calibrated speedo.

1958 rpm @ 70
divide by .69 (OD/Off)
1958 / .69 = 2837 rpm @ 70

35's that measure a true 35"
1902 @ 70
2756 @ 70 od/off

35's that merasure closer to 33"
2017 @ 70
2923 @ 70 od/off

It's just numbers, no magic there...

 

Yeah my tires measure just a hair over 33. Looks right to me!

 

35's, 3.8 and a auto. You will want 5.13's. I ran those with my '07 auto with 35's and it was perfect on the highway. If you decide to go with 5.13's and want to keep costs down a bit. I have some 5.13's for sale. Check this thread out.
https://www.jk-forum.com/forums/jk-a...r-sets-340808/

 

If you have a d30 front axle, I believe 5.13's are as deep as you can go. I run 5.38's with 35's and I'm happy with the rpm range it puts me in on the highway. You would probably get a little better mileage with 5.13's though. As for 4.10's, I had them stock with my 35's and there was no motor left on the highway. Even the slightest of incline in the road would make my transmission downshift. Just my .02.