It's a definite maybe. Too many factors. If youÆd have very powerful engine than it would prob be true. On somewhat less powerful, at least considering power/weight ratio, it would depend on other circumstances like driving up the hill, speed, larger tires etc. It could quite possibly be easier on the engine (meaning less throttle), to drive with lower gears then higher leading in some cases to possibly better mileage. Unfortunately this kind of swap will prob co-inside with larger tires, lifts and other nice perks of 4wheeling rig. Those will have much greater deteriorating effect on mpg then higher engine RPMs by itself. We are only talking about 15% change in gear ratio, and correspondinly RPM. I wouldn't expect that to have significant effect by itself.