Don't forget that the jeep is less aerodynamic than a brick!

On the highway, there is a mpg limit you will hit no matter how efficient a motor you have.

On the trails, bouncing over rocks, creeping around in 2-lo, and etc. I find that I give out long before the gas tank does.

My opinion, pick a motor, then gear it to match. It's not going to make too much of a difference in the long run.

PS: Going down hill from vail pass, I have to give it gas to keep up to the speed limit, thats pathetic aerodynamics