So i found this:
Quote:
According to the experts at TireRack, the tires on xdrive cars can be different sizes from original equipment (but must be within 3% of original tire diameter), and they need to be within 1% (not to exceed) diameter size of each other to not cause problems.
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That works out to just about 4/32" difference in tread depth allowed front to back. I don't believe the 3% of OE for overall size as long as they fit and are all the SAME size or within 1% rolling circumference.
That is for x-drive. i think the pre LCI can have up to 3% difference front to back axle.