Some threads where the DSC lights came on due to differences in tire sizes. Two of the threads mention that the BMW spec is 3%.

http://www.xoutpost.com/bmw-sav-foru...xed-tires.html

http://www.xoutpost.com/bmw-sav-foru...-inactive.html

http://www.xoutpost.com/bmw-sav-foru...er-thanks.html

http://www.xoutpost.com/bmw-sav-foru...ize-tires.html

There are quite a few more.

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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