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I think you will find that the speedometer laws dictate that the speedometer must not ever show that you are going slower than your real speed. This is the priority for the lawmakers and manufactures depending on country the usual rules are 10% + 4km/h overspeed allowance.

The second part of this is that they have to be designed to show a higher speed than you are actually travelling when new. As your tyre wears down to a smaller diameter it will spin faster for the same speed, the speedo should get more accurate as the tyre wears, then go back to reading ‘fast‘ when you get a new tyre.

Using another vehicle as a reference is not a good idea, as they run under the same rules.

If comparing to GPS speedo then the GPS needs to compensate for altitude and gradients, and not be shielded by buildings/canyons/trees, etc. If it can ‘see’ 7 satellites at the same time, this is good. 95% accuracy is a good achievement.
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