JK Unlimited with Auto Transmission and 35 inch tires.

Daily Driver with mild weekend wheelin' (sand, beach, forest trails, etc...).

 

Test drove a 2 door JK with 35's on a 3" long arm kit and 5.13 gears and that was dreamy. So i would say 5.13's

 

Something else... I had a local auto shop tell me that if I put 5.13's in there that there would be problems with the 3.8 engine overheating due to the increased sustained RPM at highway speeds?

I've not heard of anyone complaining of it?

 

Your rpm's would be the same if you're running in 3rd with the overdrive off. That's how I run on the highway and I've never overheated.

 

Ive had my 5.38s for about 2 weeks and i love them. at 70mph on freeway Im at like 2600rpms no more shifting up and down on the freeway and I am able to climb most hills without downshifting

I LOVE THEM

I did 5.38 because i want to leave my options open to go bigger later.... if I were going to stay with 35s i would have done 5.13 for better highway mpg.

by the way with my stock 4.10s I was getting about 14 mpgs and with the 5.38s i am getting 15mpg

if you dont have a rubicon or dana 44s 5.13 is the highest (lowest) you can go. i would not hesitate to do it it is the best $$$ ive spent on my jeep.. the low range rock crawling rules

 

Take a minute and do a quick search. You'll find about 1000 threads asking this same question. The overwhelming answer for an auto with 35's is 5.13 gears.

Open the Modified FAQ's and check the rpm chart. (plug in 'actual' tire heights, not mfg's spec'd heights!!)

https://www.jk-forum.com/forums/show...276#post211276

You'll notice that there really isn't much difference in the rpm's between the two ratios, but the 5.13's will allow you to hold speed better, which ultimately improves mpg's since you are not lugging and/or dropping down out of overdrive as much. Also note that this engine likes to run in the mid 2k's.

Can you get by with 4.88? Of course. And many people opt for this ratio. It's a personal preference. But, ask 100 people with 5.13's if they wish they had gone 4.88 instead, you will find very few. Ask 100 who went with 4.88 if they wish they had gone 5.13, you'll get a much different answer...


Also, there is a dyno chart in post 4. (I know dyno results can vary quite a bit, but it's good for a reference)
https://www.jk-forum.com/forums/show...-stock-Rubicon


And for the locker, are you looking for front, rear, or both? Any specific budget? Any reason for going autos vs manuals?

 

I did the search and even read the FAQ files before posting. According to the chart in the FAQ, even the stock tires are undergeared with the stock gears and the auto transmission. That leads to my suspicion of the data.

The locker question is for both front and rear. No particular budget. An auto locker makes it a bit more of a no-brainer. Just hit the gas and go. Fewer things to go wrong with it, etc.

I hear the Aussie lockers are pretty good as well.

 

True statement. Factory final drive ratio on the auto's doesn't suit this engine.


The Aussie locker has a good rep from what I've read here. And the price sure beats the other two. (No rear option though, the last I heard) Can't comment on the others in a JK, but on my dodge, I ran a tru-trac up front and a detroit in the rear. Took some getting used to on icy roads, but offroad performance was great. For the JK, I'm leaning toward an arb or elocker up front and a detroit in back.

 

Rolling A 5.13's with ARB lockers. The gears are one of the best upgrades I have made. Would have gone 5.38 if they offered them for a Dana 30

 

Find a new auto shop QUICK.

A stock rubicon 6 speed turns more RPM on the highway than an auto on 35s with 5.13s. If you shop were right there would be a lot of overheating rubicon 6 speeds.

Also in both cases (high and low rpm) roughly the same amount of work is being done to move the vehicle at a given speed. With steeper gears that same amount of work is split up over more revolutions. That means less heat per stroke. It also means lower cylinder pressures. Lastly it means more water flow and more oil flow for the same amount of work.