I've got 17x10.5's with 315's on mine. Seems like 295/35R/18's would be the largest as far as width goes. Any bigger and it might not fit that good (or look that great) on the wheel. Depending on brand, you might find 305/35R/18's that may work. 10" is the bare minimum width for 305's on the tire calculators that I've seen over the years. You should be good width wise as far as rubbing in the back with 18's, as long as your Mach isn't slammed. If it's pretty low, you might as well roll those rear fenders. You also may need to loosen your passenger side exhaust after the midpipe, if it's aftermarket. Mine was rubbing badly on the tailpipe on the passenger side. I just repositioned the exhaust a bit and made it work. I've got about 1/4" between the tire and tailpipe of my Borla Stingers.Ok cool! Do you know the exact size?