I had originally bought 315's and put them on a 10 inch rim out back, due to someone who didnt know what they were doing. They looked weird, squashed, and down right ugly. Checking into the situation 315's cant go on 10 inch they need 10.5 or even bigger. So....getting a 10.5 on the rear and depending on the offset of the rim, the edge of the tire is going to definately extend past the top of the fender. I was told 285 is the largest you could go with the 05 mustangs without flaring the fender wells. My car hasnt been lowered either (which I want to do) but that will come into effect as well. I have 285's now and no problem.