Depends on the ratio of the overdrive gear. The reason the 4.10's get the bad rap about mileage is because the RPM's are higher at cruising speed. Naturally that will burn more gas. Therefor is you have a higher ratio overdrive ie .60:1 which is the common ratio on the 6th gear of the F-Bodies t56 if Im doing my research properly, as opposed to the .78:1 5th gear ratio of a t5 on the mustang. The 6th gear will offset the extra RPM's caused by the 4.10's.

As for city driving, my friend had a 97 GT that he only drove around in town. When he went from the stock gears to the 3.73's he actually got "better" gas mileage. Lower gears give a car more mechanical advantage and the engine doesnt have to work as hard so you dont have to rev as much to get moving.