Once you know the correct speed as per GPS device, it can't be that difficult to drive at 33 for 30, 75 for 70 etc.

Oddly enough our Outlander speedo is the most accurate one not fitted to a lorry i've ever known, on half worn tyres it shows 70mph when the actual speed is 68, when the tyres get replaced i think it will be exact.

Lorry calibrations are very accurate, tyres are measured and the speedo is calibrated exactly, obviously its forst claibration when new is on new tyres, by the time they are recut the actual speed will be 53mph when speedo is showing 55.

Calibration of speedometers on police vehicles is only for those which are used for speed related monitoring. It is not available to the public under normal circumstances. It may be available from specialists at high cost. If the police use a speedometer for speed related offences it must be checked for accuracy which can be using a combination of stopwatch and a measured mile as mentioned by Collos25. As also stated above the speedo must read high or fast by law. In normal motoring conditions it would be unwise to have a speedometer that was supposedly accurate as that would give you no room for error at lower speeds thus making it possible for you to exceed a speed limit. If you have checked your speedometer over long distances on the flat at constant speeds via GPS (short distances can be unreliable) then the simplistic answer is to accept that difference and drive accordingly. If however your speedometer is reading slow then you need to get it replaced.

The measured mile is always a public road for all to use with a stop watch it is easy to find out if the speedo is correct or under or over reading instead the OP uses a GPS which depending on the algorithm installed can be pretty inaccurate on other than flat land.