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.
Similar questions
On the occasions I use my smartphone as a sat nav, I get a readout of the speed of the car on the screen. It always shows a slower speed than my speedometer by 3-4mph. Since this can make all the difference...
Cars made before 1932 do not have to have a speedometer. Are there any special dispensations for these cars when they pass through camera controlled average speed sections of road? GPS speedos have issues...
The speed recorded on my satnav is 3mph slower than the speedometer reading, which is likely to be accurate?

Ask Honest John

Value my car