I know that the 1991 and newer Mustangs have a larger wheel well than my 89, and I have 245/45/17s on it, and they do rub. I am at stock ride height though. I have had 1991 Ponys with 225/55/16s rub also. I think you should be fine with an inch but any lower may have issues.
Mine rubs on the suspension, not the wheel well though, just to let you know!
it's hard to guess if it will rub or not. the backspacing of the wheel, how much you lower your car, and how tall the tire is all affect it somewhat. i had 93 cobra wheels with 245 45 17 on the front and they only rubbed when i turned the steering wheel almost lock to lock. the rear rubbed a tad here and there with 275 40 17's.
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