We drive bricks powered by tractor engines with parachutes on the back, they aren't going to get good MPG.
I traded a 3/4 GMC long bed, big motor when I got my Jeep Rubicon. It got around 16 mpg. At the time, I asked D.J., my salesman, about the gas milage. He said, "Doc, this thing has all of the aerodynamics of a brick." I get around 15 mpg after raising the body another 2 inches and adding better tires. I gained about a half mile per gallon with the tire change, which I assumed was due to less road to tire friction.
I added racks, heavy duty bumpers, etc., so It's probably 1,000 pounds heavier than when I bought it, and it gets poor milage, but I have a blast driving it.
If you're looking for great gas milage at this time, don't buy a Jeep, but if you want a great ride, buy the Jeep. I have people stop me regularly to talk about mine; where did you get the racks, Gobi, http://www.gobiracks.com/Jeep/JKL.htm
, or the bumpers. I tried to get BESRK to build those, but he wasn't doing bumpers for Rubicons, so I bought Garvin, and have been very happy with them.