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
![](http://images.honestjohn.co.uk/imagecache/file/pad/120x120/media/16477727/537450_458533677529584_1473549666_n.jpg)
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.
Tags:
speedometer
tyres
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