I have a 2012 unlimited with 3.73's, 3.5"lift and 35's. I've seen the gearing charts posted on the forum and I'm right in line with the chart. At 70 mph Im a tick over 2k. What I'm trying to understand is people are talking about re-gearing to 4.56 or4.88's but, according to the charts at 70 mph the jeep is tacking about 2500 rpm's. Doesn't higher rpm's lower your gas mileage? Why would I want to go from 3.73's to 4.56 or4.88's if I am going to get worst gas mileage . Don't flame my to hard if this seems like a dumb question, I'm just trying to understand. Thanks




P.s I have an auto trans.

 

Yes, by going to a lower gear ration you will loose mpg, but you will gain hp at the axles. When you moved up to the 35" tire you lost hp and regearing helps reestablish it back.

 

These engines don't have enough power at those lower rpms to efficiently run a non aerodynamic jeep with heavy 35s. So theoretically you're thinking is right but there are more factors you need to account for.

 

I don't think it's so much that you'll "gain" HP so much as your engine will be revving higher and be in a better HP range.

 

True that more mixture runs through the engine at a higher RPM, but if the engine has a tougher time pulling the load at a lower RPM, then the computer will dial in a richer mixture and less advanced ignition timing. This would result in more fuel consumption per mile than an engine running a little faster with a lean mixture and more advanced timing.

With 35" tires, the engine is for sure pulling more load than the factory tune was designed for. That's why when everything else is equal, a regear that results in the same highway RPM as stock can return better fuel mileage on bigger tires.

But, everything else is not equal because the bigger tires have more wind resistance, wider tread, and a mileage-robbing tread pattern. Still, a regear will probably get a little better mileage and be more fun to drive.

 

so the sport comes with 3.21's with the option to upgrade to 3.73's( which I did) so does that mean that the sports computer is set up for 3.21 gearing and I gained some back by going with the 3.73's? My jk with 35's feels a lot more powerful then my 2002 tj with 32's and the 4.0L. So with the pentastar and 5 speed auto would it be worth going to 4.56's, anf if so, how much worse will my gas milage be. Right now I average 17.5 mpg with mixed hwy and city driving

 

If the RPMs from your stock gears are so low that your automatic transmission is frequently downshifting just for you to maintain speed (esp at highway speeds), that can have more of a negative impact on your mileage than consistently running a higher RPM (after regearing), but not having your transmission frequently downshifting.