worst case for my 99 ZJ 4.0 11100 miles stock is in the snow, at 4 mpg city. in the summer on the freeway i get up to 22 mpg. but thats on a good drive 200 miles+. but there are many factors involved in the gas mileage of a vehicle. rings plugs valves gear ratios timing tires torque converter in autos. the list it huge. the only thing i can really tel you to try is to try and keep it around 1750 rpm freeway. the least amount of pedal is best. i can get from 10 worst to 18 mpg best on a 10 mile freeway trip. depending completly on how i drive. but if that dosent give you noticeable mileage increase then i say you should check some of the stuff i listed.