I thought my 27.4mpg (almost all city) was bad since I do love a bit of acceleration from every light. Just added the bigger wheels and still am a bit heavy on the throttle but I only dropped down to 26.9 and this is with the AC on almost all the time. I also live at 5800ft we even lose a bit of ponies up here making our engines work a bit harder. I don't see how 20-21 is even possible when I get that around town with the AC on in my STI.
If it doesn't correct itself take it to the dealer.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch