If you wanna see if you've got a buggered maf, unplug the maf and see if the car drives any better.
As far as I know, 17's or even 18'2 shouldn't affect your MPG that much.
It could be a series of things, possible boost leak, well more thans likely a boost leak.
Get the car scanned for starters as this will give you a good idea of what's wrong.