Yesterday I had my navigation system displaying the speed I was travelling at. So I could estimate the real speed compared to that displayed on the tachometer.
Between 80 and 140 km/h the tachometer displayed about 7-8 km/h too much. But on one stage of the autobahn I accelerated up to 190 km/h on the tachometer and the navi showed 188 km/h. This is really strange. The faster I drive the more precise the tachometer becomes. Whatever that means