I can tell you from experience that you can't order a Reserve with the smaller tires - I tried. Will a dealer swap with a basic or select he has on the lot? Maybe. Is it safe? I'm pretty sure it is but your speedo may be off a little. Regardless of my preference for the 18's I really like and am enjoying my Reserve with the 20's. Talk to a dealer and see what they'll do for you. Good luck!