huygens

I am finding that charts and opinions from sales sites and salesman seem to more agree with what is in their inventory that what I am reading. What say you die hard JK enthusiasts?

2010 JK Rubicon with 3.8 (dog engine), automatic transmission, and 35" tires. What gears would you put under it and why?

Currently it has the 4:10 factory gears. It is not my primary driver but I do use it as a backup driver so it does need to be able to do reasonable highway speeds, but its primary role in life is a weekend toy. Charts show as low as 6.00+ while I have has sales people tell me everything from 4.88 to 5.38.

What say you?

huygens

For reference here is the 3.8 chart I was referencing.

https://cdn1.bigcommerce.com/server3...g?t=1411683059

karls10jk

You've got other things to consider with your decision. What's the weight of the vehicle look like? If it's heavier then you'll want to go more aggressive on the gears. If you plan to run larger or heavier tires then you'll want to go more aggressive. If you will be in a hilly area, you'll want to go more aggressive.


I've got a 2010 3.8 as well with 35's but 3.73 gears. She's not fast on the highway and requires downshifts but it's not terrible. I'd like to be at 4.10 or 4.56 so with the auto you'd want to go one set more aggressive, so a 4.88 would probably be a good spot for you. I've heard that moving just one set of gears isn't worth the expense unless you're adding lockers or there's some other reason you're in the diff, that's what's kept me from going 4.10's and I feel the 4.56 would be too deep and start taking a toll on my gas mileage.


For a point of reference, I've got 200k miles on the Jeep, 50k of it is on 35's and most of the rest is on 33's. I drive mostly flat lands though so I can get away with less aggressive gears.

huygens

Very valid points. Texas Hill Country and a lot of steel bolted on (bumpers, fenders, etc...). Wasn't great with 33" tires and 4:10 but was tolerable. Now with 35" tires it is a mess, always downshifting at any highway speeds (60+) and has trouble climbing over stuff it didn't have problems with perviously when in 4 low.

nthinuf

Lots of personal opinion involved, and lots of strong feelings that the particular ratio someone is running is the only ratio you should consider. Read as many different opinions and reviews as you can find, it's expensive to do it over...

That said, I think these 3.8 auto's can use all the rpm (hp) they can get. I personally would not be happy with 4.88's, so will suggest you consider 5.13 and 5.38.



My experience: I have been swapping back and forth between the hilly areas around Austin and along the front range/mountains in Colorado. Went to 5.13 while running 33's. Then up to 35's for a few years. Then regeared again to 5.38's (as part of a PR44 swap). Then finally went up to 37's a few years after that. Both 5.13 and 5.38 were great with the 35's, and I even thought that 5.13's with the 33's was perfectly fine - (though arguable from a strength perspective considering they were in a d30.)

And as a final thought, we generally get someone chiming in about "screaming high rpm's", so double check that chart and maybe you can prove or disprove that statement for yourself...

huygens

If you are running a 3.8 in the hill country around Austin then I suspect we have waved at each other a time or two. This tells me a lot. Thanks for the feedback.

resharp001

You can't undergear that 3.8L. I'd say 5.13 since you have the 4:1 TC. If you have any inkling of wanting 37s, 5.38. Also, I don't have the 3.8L, but my understanding is that engine is very forgiving to higher RPMs. The 3.6L on the other hand.........it sure likes to guzzle the gas up around 3k RPM!

Schlieko

I have same year JKU. Did 5.13 and have been very happy.

huygens

Schlieko, what tire size do you have and is yours a Rubicon? Is 80 a reasonable highway speed to expect for the highway and how does it do crawling?

RodCyn

I have a 10" JK (2dr) Rubi/automatic. I had 5.13's with my 35's and now am running 5.38's with (heavy) 37's and bead locks. This seems to be the right combination with my driving style. Many times 80 mph is the norm. If you plan to up your tire size, then I'd suggest you go to 5.38's, otherwise, 5.13s' should suit you well.