Thank you John for bringing in the gps measuring principle.
GPS itself is not hat accurate. It can have a 1.5m inaccuracy with a standard receiver.
It than measures straight lines in a frequency of 1 Hz.
So, when you travel a straight motorway and the algorithm calculates a couple of iterations before displaying speed, it is quite good.
On curvy roads...
So please do not overestimate the accuracy of gps speed.
On the other hand speedometers have improved a lot over the latest years;-) So it is often not possible to calculate an extra 10% as the Mercedes example shows.
Also the rev calculation depends on pressure as well as tyre wear.
If you travel exacly to the point - you should be on the safe side;-)
