My 2003 S speedometer reads high whether its with the standard size 18 inch wheels and tires it came with or the standard size 17 inch wheels and tires I bought later. I asked my independent Porsche garage about it. He said they all read high. He also said Porsche does this intentionally because there is a law in Germany that would subject the manufacturer to a fine if even one car's speedometer said less than the actual speed. The fine is an amount per car that is then multiplied by the total number of cars of that model produced. I'm not sure I buy that. But I have put the Speed Box app on my iPhone - there are lots of free speedometer apps for smartphones, and it shows my Boxster's speedometer is a bit optimistic! Here is a screenshot of the highest speed I've ever recorded -
If you're still reading this screenshot was taken on the Bullet Train in Japan, not in my Boxster.