I have a 93 YJ, 4.0 automatic. When I bought it in 1996, it had 40,000 miles on it, 3.07 gears, and 235 75 15 tires. It would do an honest 20 MPG on the highway, 17 around town. I once drove from Maine to Pennsylvania without stopping for gas.
It does not get that mileage anymore, right now it gets right at 15 MPG with 210,000 miles on the clock, 3.73 gears and 33 inch TSLs.
The OPs milage seems low to me, it should do better than that.