I have a 2004 Audi S4 cabriolet. I have after market wheels and regularly drive to Austria in winter for skiing. On the original 18" Avus wheels I have bridgestone winter tyres which I swap onto the car for the trip. You don't need 17" wheels to fit winter tyres, this refers to the clearance around the back of the wheel when fitting chains, not tyres - a problem which is avoided by using an alternative to chains called "Spikes Siders", these clamp onto the outside of the wheel and go over the the tyre without the need for going around the back of the wheel. They are a lot easier and faster to put on than chains. In anycase, the wheel size is does not refer to the use of winter tyres.