It's gonna lower your fuel mileage, depending on how your drive. I'd think it'd be the same concept for jeeps as it is in the wold of sports cars. But when you add gears what'll happen is(just throwing random numbers out) you cruise around town in 3rd gear at 35 MPH, well what ever your current RPM is with stock gears, it'll be quite a bit hire with a gear ration of 4:88, thus not as good gas mileage, but of course you could just put it into 4th. Like I said, just throwing numbers out there, I have no idea what the speed to RPM ratio is on a jeep. You'll have more get up and go but a loss in top end speed.