I have 17" and have had like 7-8 flats. So I've put on TONS of milage on my factory spare, even would drive on it at normal speeds. I got the car used when it was 4 years old, its 10 now, so who know actually how much road that spare has seen.
Anyways, I kept my stock 15 wheels. So I thought one time Id use that as my spare. When I got my real tire repaired, I noticed that I could not fit my 15" in my trunk. I mean, I could, just not where the stock spare sat hidden. We've done this with a mercury capri. When it got new wheels, we kept one stock wheel as a full sized spare and it fit. Weird?