As you increase the wheel size, you decrease in sidewall size. Thus, the overall diameter of stock wheels should be equal to the new wheels, if you did your math right.
If Justin got bigger wheels, his sidewalls would be smaller, so that the overall diameter of the wheel was the same, or very close to the same.
That's always the general principle used when changing rim. Whatever the rin changes to, there has to be a sidewall change in the opposite direction to keep the speedo happy, because it reads off of overall tire diameter.
/class
Yea I understand that concept already I guess. What about people who think it's cool to put 20's on their car and the diameter is wayy off?