@Ghostbiker, does the thou have the speedo driven from the gearbox then? I thought they were all still driven from the front wheel and the dreaded plastic lugs of mass breakability.
Or did you put the sprockets on the wrong end of the bike?
(easy mistake after the pub...)
In a straight line at constant speed satnav should always outdo a speedo but will never out brake or out accellerate a bike due to slow update/calculation times, the accuracy readout of satnavs is just an estimated guess, to know how far out it was it would have to know exactly where it was to start with so would be 100% accurate, then you start getting into the technical side of waas/egnos/differential gps...