As far as I know, all car speedos over-read your actual speed but my GTD is far closer to the real speed (as tested by independent GPS) than any other car that I have owned.
In my Mk 6, the over-read varied depending on the speed that you were doing but at 74 indicated, you were really doing 70.
On the Mk 7, it seems to be a constant 2 mph over-read, regardless of the speed that you are doing.
Has anybody else tested their car to see what it does?