Good question and thanks for link Wrighar.
I fit winter tyres, my summers are 18's and my winters are 16's (Saab OEM alloys). According to the 9-3 owners manual it does state what size summer and winters you can fit to your car, it shows both rim size & tyre dimension. But be careful, if your fitting SAAB OEM rims and sizes are within the guide from the manual then you are OK. But.. if you fit aftermarket alloys then that is classed as a mod.