I get 15MPG with 4.56's and 35's but then again I tend to drive sprinted. Hwy speeds are 70MPH so a minimum is 65MPH. If you go slower you become a road hazard. The biggest problem at those speeds is wind resistance. If I slowed to 50-55MPH, I bet I could get over 18MPG. If I drive more relaxed I can get over 16MPG. I'm mostly doing short city driving with a few hwy stretches on the weekends so my mileage is low. When I was doing a 40 mile commute on the hwy, I'd get 18mpg. I also tried putting the jeep on a diet and removed a couple hundred pounds of weight but that didn't change the mileage much. Which lead me to believe it was more about the aerodynamics of the Jeep and the wind-resistance at those higher speeds.
Going with lower gears does not necessarily mean your gas mileage will tank. If you're running too high (numerically low), you're lugging the engine all the time and that will use more gas as well. Keeping the engine in it's power band is more important.
