Gasoline is a fasinating subject. Here is a link to a lot of information about it.
Gasoline FAQ
Now back to the subject of this thread.
In any given year/model, the 4 cyl Jeep will get slightly better city mileage and the 6cyl will get slightly better Hwy milage.
The average mileage will be about the same. I would get a 6 cyl and enjoy the extra power. Another factor to consider is a 4 cyl with a 4 speed manual transmission will get better mileage than a 5 speed. On the other hand, a 6 cyl 5 speed will get better mileage than a 4 speed. (with stock gears and tires) <img src="/forums/images/graemlins/cheers.gif" alt="" />