They are in fact 18's. Race rubber, as far as I know, only come as big as 18". Yes, even the world challenge series run on 18's. Everything runs on 18's.Yes, you're correct. You want to have the minimum amount of unsprung weight. So you want the wheels to just barely clear the brake calipers.18's is what the model 3 comes with. And that's what I'll run when I do place my order.