One thing you notice, that almost all (except my old Fiat, Police gave me a nice proofing picture of it) speedmeters show some 5..10% more speed than actual. In my car, I took the time for doing 120km/h for "exactly" 2km (marks at highways in switzerland). The actual speed was 112km/h

With my new car, there is an average speed function. When I reset this at constant speed, this unit shows much more the correct speed, than the tacho.

One thought: tire pressure differences, and also tire abrasion might make a difference of up to 3cm (??) in diameter, this might cause errors up to 5% (??)...

Werner

PS friends of mine have developed a system that transmits tire pressure wireless to receivers attached close to the wheel. I think french cars offer this option now. But it was never used to correct the speedometers but to warn if pressure was too low or too high.