Twiztga

I would say the 285 won't make it; I've got 18x10 Roush wheels all around with a 275/40 in the front and they barely cleared the struts, we are talking 4mm..

ShawnAlexander

I just proved that you can put larger wheels and tires on the new Mustang. You state that the 275 is close to the strut. Well, you need to adjust the offset.

Nuke

I have 275/40/18's all around on the Cobra SVT's (very similar to the GT500's) 18 x 9.5 with I think a 48 mm offset. Lot's of room on mine but my suspension is stock. However, if I had to do it over again I'd go with 255's up front. The 275's track every little uneven section of the road such that on a heavily worn road with tire ruts, I'm fighting the wheel a lot more than I did with the stock 235's. It can be a pain in the butt sometimes.