I can't really say what the widest wheel and tire combo possible is, but I can tell you what I have/had on mine. I used to run a set of 295-50-15's on 8.5" wide rims, which was fairly close, but did not rub. Currently I have 255-45-18's on 9.5" wide rims, they do not come close to touching. I really should have gone with 275's, I think, but wasn't sure if they would fit well on the rims. Of course, the wider you go, the more attention you have to pay to backspacing. Also, my mustang is set up with a little rake, and is not sitting low in the back like a lot of them do after 40 years. You could probably go a little wider than I have, but not much before you have to modify the tubs.