Check out fueleconomy.gov, they have factory numbers along with user numbers. If I remember correctly, the numbers flamingpony posted are about average. I wouldn't get one as a dd personally. My 96 Cobra gets 20 in town (worst I have gotten is 18, and that was with several passes at the drag strip and a heavy foot). I have gotten up to 27 on the interstate/highway. Not a big mpg difference, and better resale on a V8, plus more parts available.
1996 Mystic Cobra #1219
H&R sports, Bilsteins, MM c/c plates, King Cobra clutch, FRPP 3.73's, MGW Shifter, 80mm Pro-M mass air meter, 9mm FRPP wires, Mac underdrive pullies, Flowmasters, Mac o/r H-pipe, Sunroof, smoked head/fog lights, envoy fog light swap, OEM 17x9 04 Cobra wheels