he he, first things first, its a JEEP! ford makes trucks!
you should be getting a little bit better gas milage though. im not sure if youll get 15 in the city with that being an 8 cylinder. the biggest determining factor will be the milage on the JEEP, and whether or not you have a lift and how big your tires are. if its stock and under 80,000 miles i might be a bit worried. but if your lifted and running mudders on an engine with 120,000 miles on it, then no big deal.