Personally, I would put them on the front; as noted above, that is what you depend on for steering and braking. Actually, I would buy four, but sometimes that isn't practical.

Tire Rack and other tire stores will tell you to put them on the back. That is because they are more concerned about oversteer than understeer, and worry about the liability they incur if a vehicle breaks loose in the rear with insufficient tread. I think that the inclusion of DSC and ABS negates that risk to a large degree.

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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