Which part isn't clear? Anything beyond approximately 3" [EDIT - 3%] difference in rolling diameter front/rear stresses the transfer case and risks damage. With a pre-2004 X5 solid transfer case that shows up in problems with the splines. With an x-drive transfer case in either an X3 or X5 that shows up in wear to the clutch packs. Different manufacturers of tires have variations in actual rolling diameter, which is why many tire stores will want to replace your tires four at a time, not two at a time, so you don't come back and blame them if you have problems with a transfer case later.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue