I just put on my 17"s and have been a bit heavy on the throttle and have dropped 1mpg down to 26.3.
If I was getting below 24mpg I would be concerned;this is a 2800lb fwd 160hp car compared to my STI which is about 3600lbs and 340hp and it's driven hard occasionally and gets 20 with almost all city driving.
Are you going by your computer calculated mileage? I go by mine to monitor any odd drop or something but I still use good 'ol math and on paper I'm getting closer to 30 so it's off by 3.7mpg which is a lot.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch