My speedometer is +5kms out

[brad]

And not if your wife did it without telling you.

those of us that like accurate speedometers also arrive at meetings on time.

In this day & age of time specific speed zones only a fool would have a time piece not set correctly.

[wai]

It's not simple, and is affected by the tyres if they are not as per the tyre and rim placard.

This is the relevant part of the ADR

5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test
speeds specified in paragraph 5.2.5. above, there shall be the following relationship
between the speed displayed (V1 ) and the true speed (V2).
0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

The paragraph 5.2.5 refers to the speed ranges the tests must be carried out at.

The key is that it must never indicate a speed that is slower than you are actually travelling.