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.
Similar questions
From 1926 to 1930 under a nationwide 20mph speed limit, vehicle numbers rose by 35% but deaths rose by 50%. From 1930, with no limit whatever except 30mph in towns, until 1938 (the last full year before...
Politicians do not think like ordinary people and sadly they are rarely "honest". The intention to introduce blanket 20mph speed limits on residential roads and 50 mph limits on all single carriageway...
You are, it would appear, always right. "Smug of Suffolk" here has just been snapped doing 54mph in a 40mph zone - I have been driving 35 years without an incident of any sort. I was keeping up with...

Value my car

Save £75 on Warranty using code HJ75

with MotorEasy

Get a warranty quote

Save 12% on GAP Insurance

Use HJ21 to save on an ALA policy

See offer