Mr J
Respect to your Einsteinian reply - I'm a bit quanderied then, assuming that the GPRS is accurate - does that 7.333% equate to the diff between the original tyre diameter/speedo, when the speedo was really showing around 5mph too fast, in true speed? Obviously not across the whole speed range but average around 75 -100mph.
I know this overread is true of most cars and most model ranges now have a big wheel option. It seems strange that the calibration isnt based on the standard wheel size. Obviously there is benefit to the manufacturer if the car seems a bit faster than it is, but that differential alone could tip you over a radar camera/gun limit.

MM