I have a Titanium with 18" wheels (suspension package).
When I first bought the car, it was getting much lower miles per gallon that I thought it should. ~24 mpg in the city and a best of ~34 mpg highway. As the engine broke in, currently just over 6,000 miles, I have noticed that city mpg has increased up to 29-30 mpg. I still have not bested the current 34 mpg on the highway, but haven't taken a long trip like that since. I did drive ~250 miles yesterday, but I was not driving for max efficiency, only netting 33 mpg. Driving at 90-100 mph is not the way to get max efficiency.
Keep the faith, as your engine breaks in, the efficiency will improve.
2014 Mustang GT custom ordered to replace the GT500: Sterling Gray, NAV, Recaros, Brembos, Glass Roof, back up camera, 6 speed auto, loaded
2012 Focus Titanium - 18" wheels with handling package
2009 GT500 Shelby (700 rwhp) - sold