Then I had to face them in real life - like I mentioned before, three tires were 200/60R15, the fourth was 195/65R15, and the spare was 200/60VR390. The question I asked myself (being, as usual, overly paranoid) was - how the hell do I know if the wheels with R15 tires on them are indeed R15 wheels?
Two answers:
First, gotta look at the markings on those wheels. Sometimes, you have to take the cover off, sometimes not:
Second, compare them against known BMW wheel styles (either look it up in a catalog or search if that link is broken).
And, of course, nothing works better than a asking the question on mye28.com - just make sure you do your homework first. Sometimes, if you're really thorough, you won't even have to go there :)
