I have a 2000 Jeep Grand Cherokee on 35 12.50's. I want to regear it so it gets better gas mileage, whats the best gears to use and how much would it help my MPG???

 

Well, it would depend on a number of things, what transmission do you have? what transfer case do you have? what do you use your Cherokee for ,DD weekend wheeling, or mostly a wheeling toy, or a wheeling only rig?
I like low gears because my Cherokee is mostly for wheeling, so I don't put lots of highway miles on it and mileage means next to nothing to me. If it was my only transportation mileage would be way more important.
I'm running 4.88s with 35/12.50s and get about 15 mpg with an AX15 manual trans, when I get a tank full without being in 4x4 which is only about twice a year. [ Running in 4low in 4 to 6 foot of snow for half a tank I get around 7-10 mpg. ]

 

456s or 488. Your gonna get ****ty gas no matter what

 

Use the gear calculator at www.grimmjeeper.com