[Loz]

Just for clarity, the manual says that the speedo over-reads by an average of 8%, over the whole range. If this is true, then in theory the speedo could be dead accurate at say 30mph and more than 8% out at say 150mph.
The lap-timer is said not to over-read, that it is in fact accurate.

I can only assume that the speedo over-reads in accordance with general secret police state policy for all speedometers, that is, they must over-read our speed in order to keep us safe.