The whole trick is to just have a thin one the size of the front. The back does not really matter the size. It is the front that would have issues more then the back. Since the front is thinner, I am guessing Smartieparts thought of this and thus made the smaller of the two the thinkable option. As the poster below you said. It is just to get you from point A to B, B being the dealer.With two different sized wheels front and back how does that work?