Thanks for all of the advice on winter wheels. I got 15" Blizzak WS-50 tires on Borbet type B wheels to replace my 17" wheels for the winter. My neighbor asked me if this will affect the speedometer reading and frankly, I don't know the answer. Since the owner's manual has recommendation for 15",16" and 17" wheels, my intuitive answer is no effect. Maybe he's just jealous of my Z3 and is breaking my chops. His wife wanted a Z3 but they bought an SUV.