lots of variables for poor mileage, un-calibrated tire/speedo, your driving style, your mods, the type fuel you use or how you've changed your jeep's performance tuning and so on......
it's a Jeep - once you change it from stock your mileage will change. I've got a 2010 4dr- 3.5" lift, 35's, performance coil and wires, CAI, 4.88's, bumpers etc.... my mileage around town avg 14, freeway 18 only if I drive somewhat reasonable, no jack rabbiting, 60-65 on the freeway, and no wind! lol best mileage on long trip cruise control set between 60-65 - 23mpg hand calculated over 600 miles. worst, 8 mpg against worst head wind I've ever encountered.
At 70 my mileage drops enough I don't push it unless I'm going downhill. Right now I'm averaging 16 around town but am also using cruise control as much as possible and driving a little slower...
it's a Jeep, I point it to where I want to go and hope there's a gas station when I get there :beer: