It may seem like a cop out, but there are plenty of variables involved that make it difficult to give a direct answer. If you have the stock 7.25 or the 8.25 axle, that helps things. If the springs haven't sagged, that helps as well. If you are looking to keep the whole tire within the wheelwell and are trying to maintain a safe clearance of 3/4 inch from the leaf springs and the fenderwell lip, you should be able to fit a 275-50-15 on a 15X8.5 rim with 3.25 inches of backspacing. This assumes that the rear axle is perfectly centered in the car.