when I got my x5, i had a PPI done. the BMW tech said the spring perch on one side was worn and would need to be replaced to prevent abnormal tire wear. Not sure how true that is or if it's your problem, but it's worth looking at, I'd say. I haven't had my car long enough to know if it's causing abnormal tire wear. I just got new tires though, so i'll presumably find out at some point!

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2003 X5 3.0i, 150k miles