It has to be said that , quite aside of the legislation governing the accuracy of car speedometers ; and discussions regarding their accuracy compared with other devices - the only devices which carry any weight in law are the approved devices used by police to check the speed of vehicles .
All of your personal GPS devices , roadside radar operated speed indicators and what-have-you are NOT government approved devices and , absent a legally recognised and accredited calibration certificate , have no meaning whatsoever in law .
If you were taken to court on a marginal speeding charge and argued that you 'knew' your car speedometer was over indicating on the basis of comparison with one of these other devices , you'd be laughed out of court along with your fine and points .
As far as the law is concerned , the only indicator of speed you have is the one fitted to your car , if you choose to ignore it because you feel you know better then you do so at your own peril .
The measuring devices used by the police for enforcement have to be Home Office approved and calibrated , this applies to all static devices used at the roadside , as well as equipment fitted to traffic cars , the speedometers of which have to be calibrated at the start of a shift and at the end if to be used for speed enforcement .