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On my CTTS I currently run Hankook Ventus R06 tires on the OEM 20" wheels but also have a set of 19" wheels with Continental Sport Contact 4x4's and the ride seems about the same. I think if your only changing by 1" then the tires make just as much of a difference as the 1" change in size. Look at Tire rack tests and see which Cayenne fitments rate higher in comfort. Of course you could go with 19's and the most comfortable tires and probably make a fairly large difference in your ride.