It's really hard to get good gas mileage from a mustang. There area few things you can do to improve it 1-3mpg but its not that much.
3) If the car hasnt ever been seafoamed I'd suggest buying some seafoam. It works wonders and really cleans up the engine. There are lots of threads on here about seafoam.
Something interesting I've found... the last two tanks in town I've gotten 20 mpg lol. All this driving around in 5th at 35MPH pays off apparently. So my figures are like 20 City/22 Highway. When I first got my gears I was getting 15 city, but I've learned how to baby it and it has actually improved compared to my 3.27's which consistently got me 16 mpg in town.