Depends what you mean by winter tires. If you mean a snow tread, then as Amacman points out, they aren't built for that and they won't handle well, but go ahead, just accept the increased noise and compromised handling and run them. They will wear out more quickly. If you mean a winter rubber compound, one designed for low temperatures, then as Boston points out, they will wear out very quickly on a warm day (You can measure the wear in days, not months).

If they have a true winter rubber compound, then the cut off point for using them is usually an ambient temperature of 7C, as Brit6 pointed out.

It just seemed like there were two different discussions going on in this thread.

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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