Highly recommended to go with the same brand, tread pattern, and rubber compound, so as not to negatively impact vehicle handling and safety.

Anything over half worn is a candidate to be replaced when you replace one on the same axle, as the differential will be constantly turning to accomodate the different tire diameters. You can buy one new tire and have it shaved down, but it is generally just easier to change two on the same axle.

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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