Like Jim said, I'd take a long trip and see what your mileage is. One other thought, are you letting the jeep warm up before driving it in the winter when its cold out, or do you just drive it like you would in the summer?

Sean


99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs