Having the rare opportunity to hear various rules made by AVM's from stores across the country, all I can say is that the limit is whatever your rep says it is (that day).
GM has never, and probably will never, had a policy on what is acceptable wear only to say wear items are not covered. Where that line is drawn (and I've heard reps spout limits from 8,000 miles up to 36,000) is totally a the whim of who ever is making the call. All I can say is find out your rep's limit, what they might want to hear to justify replacement after that limit, and stick to it for the ten months until you get a new rep.
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** Rob, Editor WD&S **Help is only a message post away!
robc@dealersedge.com