18's are def for show on a Stang, IMO.
After riding on 18's here in DC suburbs I don't hink I'll ever put them on again b/c bumps, uneven lanes, pot holes, and dips scare the hell out of me. 17's ride the best IMO and I usually cruise with them the most.
However 15 Pro Stars or Drag lites ride smooth as hell and you feel like you're on a cushion when you hit a bump and it's hardly noticeable. With 18's and 35's you feel every bump, crease, crack, un even pavement, pot holes, man hole covers, debris, road kill,, etc.
15's with fats and skinnies are to hard to control in corners, naturally.
17's with 275's are the best IMO.
BTW I have 4 sets of wheels from 15's, 17's and 18's and I swap them occassionally and the 17's always win out over any of the other sizes.