I autocross with my 225/40/18" tires and have no problem what so ever. It really depends on the autox track or parking lot. If there are some trouble spots with nasty pot holes, yes, you should not use your expensive 18" wheels. Otherwise, lightweight 17" wheels with quality tires are much better for performance. You're gonna get more contact patch and stiffer side walls with a low profile tire. Plus, it's good to have more rubber in the rear depending on what car you have. Next time, I'd like to buy some 245/35/18's in the rear and 235/40/18's in the front. But I don't know about the availability of those tires sizes.