Thanks for the info. Did you get your info from BMW? I had tried reaching BMW engineers through BMW North America. I was told only the dealers have access to the engineer resources. Following that path, the engineer source said that as long as the other three tires are above spec, there would be no negative impact on the xdrive components ( transfer case, differentials) if you add a new tire at full tread depth. However, the handling may not be as good as it wood be if all tires were closer in tread depth.