MP4.2+6.0

The problematic delta is sizes for the quattro system is 2 or 3% IIRC. But, a bit of a grain of salt there given the stereotype German penchant for uber precision that often doesn't match real world. As long as you stick to the same nominal tire size among the four, unless one set is literally brand new and the other worn to nothing (which doesn't make sense real world), you won't find those kinds of percentage differences relative to the book spec. Or, for a math proof, the smallest D3 size diameter wise runs just about 28.0." Many are around 28.3". Even the 275/40-20 bump up at the high end of the range gets you 28.7". For that full spread from 28.0" to even 28.7" is 2.5%. Thus, wear within a set of a given size front to back is not going break quattro...though it may disappoint a few tire salespeople. Since you put the new ones on the front, generally they will also wear somewhat more quickly and close up the difference some too.

SWelsch

Thanks for your help. BTW, when the tire was ripped I never saw so many fault lights. Cleared and all normal after the new tires