What year Cobra wheels are they?
I had 94/95 Cobra wheels on my '86. They we slightly wider than 8" (don't remember if it was 8.5 or a little less). The factory tire size for those was 255/45. I think I had 255/55s on them, though. It was a while back, so I don't remember. I do remember that they rubbed the splash shield up front a little bit, but that was it. I can post a pic if you want.