How accurate are car speedometers?

How accurate are car speedometers and what is the effect on accuracy of changing running radius of tyre as tyres wear?

Asked on 2 July 2018 by John Hammond

Answered by Honest John
When a car is built its speedometer cannot under-read but can over read by an amount that equates to roughly 3%. The best way to check is with a speed app on a smartphone on a clear, straight, level stretch of motorway, that will accurately give you the car's speed across the surface of the planet.
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