Hi
There I have used my sat nav in my cars and the only one that reads the same speed on speedo and sat nav is my Plus 8, the Volvo V70 reads 2mph lower on speedo than sat nav, the Nissan Terrano van 4x4 reads 4mph lower on speedo than sat nav.
I know the 4x4 does read lower than actual because of tyre sizes, the speedo on plus eight is near perfect in calibration no error.So I would say Sat Nav is speed over ground and is more accurate than most production cars which can have up to 10% error below, must not be above.
regards
Phil
