Depends on the width of your rims, as they obvioisly aren't stock. I'm assuming your talking about tires on the rear, and if so pending on the offset too, most people can get some 275's on the back, but you most likely will have to flip the quad shocks and/or use spacers. I had 275's with flipped quads and spacers for a while, but depending on the brand of tire (the shape of the sidewall) they would very slightly rub on the fender. You can roll the fender to fix it, or remove the quads if you have aftermarket LCA's.
OR, you could tub it, and get some real wide slicks lol
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***