Go w/4.10's!! Seriously, at the most you'll lose 1-2 mpg, and that's with the hammer down all day! I get 22-23 on the highway at 70-80, and probably 13-15 in town. But I've got a manual and I get on it every takeoff!!! That's why you bought the Mustang in the first place, lolol. 4.10's...