Interesting discussion. The only time I have never encountered a situation where a tire shop won't mount "whatever fits" was in the case of a trailer wheel, where the shop would not mount a car tire. When you consider that many modern cars have a high-performance tire option (i.e. summer only tires vs. all-season) with different speed ratings, how does a tire shop even know what was originally fitted to a vehicle. It may well be a question of state law where you happen to reside?