I had stock 14s on mine when I got it and I put 17s on it with low profile tires, thru it off about 4-5mph but mines an older mustang. Whats on it now? 15s or 16s? Shouldnt be much of a difference with 18s like baggedon said. Maybe 1-2 mph at the most. I had to have my front end aligned when I changed...idk if you will or not.