I reset my display with every fill-up. I average 18.5 to 18.9, which is some city and some highway back/forth to work. Yes, I do stand on the throttle quite a bit. The car gets 25+ straight highway. Our (GTs) mileage is not good, until you remember what kind of power we have. Think of it this way, my GT gets slightly better mileage than my wife's minivan.
2006 GT Premium Coupe, Black, 5 Speed, STL