Reviving an old thread, but here's a good article to discuss the implications of tire width deviations. Essentially, all tires have a measured/advertised width when mounted on a rim of specified width. Say you have a 9" rim and a 255 width tire is measured on a 9" rim. You get 255 when you mount it on that rim. If you mount the same 255 tire on a 8.5" rim, you squeeze the tire and the width is actually less than 255...maybe 250 or 245. The converse is true if you mount the 255 tire on a 9.5" rim. Now it may actually have a tread patch width of 260 or 265.
Tire Tech Information - Tire Specs Explained: Rim Width Range