How accurate are car speedometers?

Some local authorities are proposing that drivers will be given penalty notices for exceeding 20mph limit by as little as one mile per hour. Does this match the level of accuracy of speedometers? I thought that they had a calibration error of plus or minus 3mph.

Asked on 2 November 2022 by Neil Littman

Answered by Andrew Brady

There is an element of inaccuracy built into speedometers. Legally, they can't under-read, so manufacturers usually configure them to over-read. That means you shouldn't accidentally break the speed limit.

Tags: speeding speed limits speedometer legal issues

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