Okay so I have a 2009 X unlimited auto trans, 4"lift, and I have the stock 3.73 gears in it and i have 35" Mickey Thompson ATZ's. I drive on the interstate Mon-Fri, roughly 60 miles a day. My jeep tends to be sluggish due to the gears. I have the superchips TD 2 on the 93 Octane tune and it still has to drop gears occasionally on inclines. I've looked at gear to tire ratio charts on multiple websites. I'm not sure whether i should go with 4.56's or 4.11's. When i re-gear my jeep I am going to put new axle shafts in it and I'm thinking about a sleeve and gusset kit. So I would love some input on which gears to go with, according to my TD2 right now I'm staying between 16-18 mpg. Any input/advice on the gear choice would be more than appreciated. Thanks!

 

You have a 2009, with a 3.8 engine, an auto trans, and 35" tires? 5.13's are the most commonly suggested ratio for you to run.

While you are reasearching this, note the engine and transmission of the person replying. Most of the threads/replies these days are based on the newer 3.6 engine, that has a lot more HP and a better transmission ratio. Their recommendations will not apply to you.

You can find some decent rpm charts in the FAQ's stuck right to the top of this Modified area. Use measured tire height, not mfg spec. And if you want to find the OD/Off rpm, just divide the number you get by .69

(For example, right now, if you press the OD/Off button, your freeway rpm's will increase to right about where you will be if you had 5.38 gears with od/on. Fairly useful as a comparison.)

https://www.jk-forum.com/forums/modi...bie-has-18617/

 

+1 on what nthinuf said there. Going from 3.73 to 4.10 would be less noticeable than a fart in the wind, and definitely not worth +/- $1500.

 

Performance wise a 5.13 would be just fine with your engine/transmission combo. A 4.88 would also be a good choice. That engine will get about the same fuel economy no matter how fast it is wound up on the freeway. The 3.8 is a lot more sensitive to heat and load than it is to RPMs.

 

I agree. The 4 speed automatic transmission has a very tall overdrive (4th) gear. It's a lot taller than 6th gear in the manual tranny or 5th (overdrive) in the new automatic. What speed are you running on the interstate? According to the gearing charts 35 inch tires with your transmission and 5.13 gears would put you at 2380 rpm at 70 MPH.

 

I usually run between 75-80

 

Then 4.88's may be a good choice for you

 

That was my thinking as well.

OP, you mentioned you have a "X" model which I believe is the same as the Sport. So you likely have a Dana 30 front axle. The largest gear you can put in that front is the 5.13 but it becomes very thin/fragile at that size. 4.88 is probably where you want to be unless you're upgrading the front to a Dana 44 or larger which has a larger pumpkin.

I'm in the same boat as you and am debating between 4.56 or 4.88. 5.13 seems like it would likely give good performance but at the risk of throwing all your re-gear money in the drain on that Dana 30.

 

Yeah I think I'm gonna look at 4.56 because I don't wanna try a 5.13 and risk tearing up the dana 30

 

Just a suggestion, but do a lot more research before making an expensive snap decision based off of the few replies you got.

People have been asking this question since jk's first hit the market almost 10 years ago, so there are plenty of threads to browse through. I don't imagine you will find many people with your setup who suggest 4.56's.

And it wouldn't hurt to look at a few dyno charts. You might be shocked at the rwhp they show running down around 2100 rpm. Then ask yourself if that is enough power to push your 4" lifted jku on 35's at 80.