Boxster Coupe GTS:

 the center-lock-style wheels, which measure 20 inches up front and 21 inches in the rear and are wrapped in 265/35R20 and 325/35R21 Michelin Pilot Sport Cup rubber.

 
This contradicts Georg's tire dimensions with a 1 inch increase in diameter.  Too bad for CGT fitment Smiley
 
Tire technology improvements are thought to be a big part of the stunning performance increases of the latest Porsche cars.  Oh that the same were available for older cars.

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Mike

Carrera GT + Tesla Roadster 1.5 + Panamera Turbo +  BMW Z8 + BMW 3.0 CSi + Bentley Arnage T + GT3 RS 4.0