Do you have the Handling package? The 18" wheels do effect mileage by probably 1-2 mpg compared to the base 16" tires.
I have a TI - non-handling package. City mileages averages 26- 28mpg and highway 34 - 38mpg. I'm averaging 31.6mpg with 14K miles on the car.
2012 Focus Titanium HB - 401A
1999 Ford Contour SVT