Question About Tire Replacements for AWD

In years past I owned several AWD "sports cars" like WRX, Evo, etc and no one ever said if I had a damaged tire I would need to replace all 4 tires (and I never had a problem). Now the BMW sales guys make a point of saying if you damage a worn tire (say one with 15,000 miles on it) on an AWD BMW you will need to replace all tires so the diameters of tires are the same (which computers apparently need to know for sensing wheel slip, etc). Is that really the case because with the cost of run flats and low profile tire damage and all that could get really expensive? One sales person did tell me you only needed to replace tires on matching axle--i.e. damage right rear you "just" need to replace the left rear at same time so the rear tires are exactly same diameter.

The salesman is right, wow, did I just say that

In Europe the MSport package comes with staggered tires which have a 3.5mm difference, so there is some tolerance within the xDrive system. A new tire has about 8mm of tread and should be around 3mm (2 if you are cheap). So doubling these numbers to account for the top & bottom tread contribution to the cricumference yield a change of 10mm over the useful life of the tire. So these are the numbers to have in your head. I say that because I do not have an authoritative answer, but I would suggest you speak with the service department to see what BMW says is the spec for the xDrive system - the sales guys are just telling you what the heard from someone, somewhere, sometime...

That said, here's what I did when I lost a tire a couple of months ago on my 335d: I bought a used tire on ebay that was the same model and had the same tread depth. You might not be lucky enough to find the same tire, but it's worth a shot.