Re: How fast I am I going???
I did a small experiment on the Motorway a few weeks ago, testing the speedo against GPS speed, (Nokia Sport Tracker) with the cruise control on, and as expected the Speedo was reading higher than the GPS, about 4mph @ 70mph
the second part was comparing average speed on the trip computer to average on sport tracker, and they where within 0.5mph.
So I reckon the car knows exactly what speed it's doing, and the error is programmed in,
as kickstart mentioned the speedo can read within a certain limit over, but must not read under, so the makers possibly just programme in an increase to keep themselves covered.
PS this is just based my thoughts after a bit of fiddling about with my new bluetooth GPS receiver, and could be complete and utter cow poo.