This is of course depending on where you live. Last month I was in NYC and rented a Mustang and drove to the Hamptons. The roads in this area are a joke, I would not want to drive with 21" wheels there.
Other than a few cosmetic scratches I had no problems with my 21" wheels on my Cayenne. Bu next time I would get 20" also for my summer wheels, because the 21" are very delicate with parking.
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2012 Cayenne S White/Espresso
Ex: 993 Targa, 986S, 986 and 964 C2