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I have a JL Sahara which I installed the Mopar 2" lift kit followed by 20 inch wheels with 35's. well I found out that at interstate speeds that 6th gear is useless and gas mileage sucks. I suspect I'll need to change the gearing in the rear end. Does anyone know what gearing needs to go in there? Any info will be great thank you.
 

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Re-gearing will help with acceleration and will give you better rpm / torque in 6th. You might find that with 35s you don't have enough power to even use 6th gear. Regearing will correct this issue. At high speed, you are probably better off with the existing gear, however most of us spend our driving time under 65mph.
 

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A buddy of mine has 35's on her JLUR with the 6 speed. The 4.10 ratio feels fine for street use, but 6th gear doesn't get much usage. If you plan on off-roading it, I would go with a little lower, such as a 4.56 or 4.88 since the low range on the Sahara is not as low as it is on the Rubicon.
 

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You realize that proper gearing will actually decrease your gas mileage further, right?
That's not always the case.

Every engine has an "optimal" RPM range. If you operate the engine above that it will decrease your mileage. But also, if you go below that range, your mileage will start to decrease. In the case of the Pentastar, that bottom end is around 2,100 RPM. If you operate much below that, the engine isn't making much torque and has to work harder (i.e. use more fuel) to move the Jeep around.

I did an experiment with a 100 mile trip when I had stock tires and 3.21 gears. I ran a loop with my manual in 6th gear and measured the mileage. Then ran it in 5th gear. I ended up getting very close mileage numbers between the two runs. The big difference was that in 6th gear I would bog down going up even the smallest grade where 5th gear let me run right up the hill without slowing down.

For a good balance between highway economy and performance, the Pentastar needs about 4.56 to 4.88 gears in the axles with a 35" tire. The deeper overdrive of the JL manual over the JK means that 4.88's are absolutely a good option. If you target around 2,200-2,500 RPM on the freeway, you'll have the right balance.

I have 35's and 4.56's in my JK now. I routinely get over 18 MPG on the freeway running around 2,500 RPM at 75. And for a lifted Jeep with flat fenders, a winch bumper, etc. that's pretty good.
 

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Running 70-75mph I get about 20.5mpg on a trip through N/E Pa - JL with 4.10's and a 2.5"lift and 315/70-17 AT's.

(Transmission is an 8sp AT with full use of all 8 gears)
 

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Running 70-75mph I get about 20.5mpg on a trip through N/E Pa - JL with 4.10's and a 2.5"lift and 315/70-17 AT's.

(Transmission is an 8sp AT with full use of all 8 gears)
That's impressive! I never saw north of 20 mpg, even when mine was bone stock...must be my driving style.
 

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I just lifted mine with 35s and regeared to 4.56, and I think it is perfect. 4.56 with 35s makes each gear more useful than the stock 3.45s on 32s. It is similar to a Rubicon with 4.10s and 33s, so you could always test drive one of those to get a feel for it.


If fuel economy is not a concern for you, then maybe consider 4.88s. Just be careful because the 1st gear is already very short.
 
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