The speedos are inherently at least 5% inaccurate (optimistic on speed) - and on both my GP800 and Fuoco over 10%.
They are deliberately designed to be thus. So thwart this subterfuge, and mark your current dial with the *real* speeds.
Like this (Fuoco) - the tapes are the 30, 40, 50, 60 and 70 true marks. Note that 80 on the speedo is only just 70mph in reality: