I have 275-40-17 on the rear of my car. The wheels are 8.5 wide and I have no issues with rubbing. If you have aftermarket lower control arms you can get away with 315-40 as long as the quad shocks are removed. My fronts rub the lower A arms just a little if I turn too sharp. It will also depend on the springs you are or plan on running. Just fyi Rocket Performance on Rt 17 in York county specializes in Mustangs.