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 1 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.
Tags:
speedometer
tyres
From choosing the right tyre to what to do when it goes wrong - we have the answers.
Tyre News And AdviceSimilar questions
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?
I sold a 10 year old car with 27,000 miles and a full service history, only to be told a week later that the car had a speedo change eight years ago and has actually covered 35,000 miles. The dealer that...
Elsewhere on 
