I've got a Cayman 2.7 with 18" S wheels and I've fitted Michelin Pilot Sport Alpins in the stock sizes (235/40 front and 254/40 rear). Although the snow we've had so far here in Northern VA has been light, there have been several days below freezing. Great tires and I've had no problems whatsoever. Make sure you select a tire with the "N" designation which means that Porsche says all the sensor and such will work with that tire.

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C.G.Mojo