Ive heard tuning your engines increases your milaage but puts more stress on your engine? sorry i ride a bicycle
Also you could work to reduce the weight of your jeep. Replace all the heavy steel exteriors and unimportant metal interiors. Dunno how much that will make a difference.
Also, a jeep really sucks at aerodynamics, remodel the body of your jeep ? :P
Also, tyres and so many other things play an important role. ask any automobile mag junkie, lot of theory on this.