What the hell do i do with my car now!! :(?

I have bought new 18 inch alloys for my vw golf mark 4... Its a 1.6 petrol. The car has now gone mega slowwww and uses tonnnes of petro due to tge weightl! I got the alloys for 650 pounds and dont want to not use them now, plus they look nice but the cars basicly useless because the amount of petrol it takes! Its gone to like 24 mpg ish, i had no idea itd have that bad of an effect on the car, im thinking of buying a tdi golf now... What other cars fit the same offset as the mkiv? Thanks