just for reference, in a 325i 2.5l engine I'm getting 22-23 mpg in a short, 2mil morning run. I live far enough from work to take the car in bad weather but not close enough to walk it every day. Hence I drive short distance and engine is cold (car is in warm garage though). I think my consumption is monstrous compared to highway which is in high 30s.
My fuel bill is roughly 100 EUR per month so it's not so bad when all is said and done. Partly the reason I sold my 320d is I drive so little nowadays.
BTW, your consumption on wide tyres will obviously be higher than on narrower lighter wheels. So you have to check against that when comparing yourself to others/factory figures etc.