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How accurate are speedometers?

How accurate are speedometers? My Toyota Auris 2016 reads about 10% lower than reality, i.e. 63mph shows as 70mph and 27mph shows as 30mph. This is unacceptable. All data on this car is inaccurate and therefore of little use. Our MPG figures are way short of their advertised figures. We find this attitude to data very misleading.

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On a new car with the correct new tyres at full tread depth it is illegal for a speedometer to under read. They can over-read by a varying amount, roughly equivalent to 5% - 10% and most over-read by around 5%. In your case, this may be due to the wheel and tyre size. There are three 'official' sizes: 195/65 R15; 205/55 R16 and 225/45 R17 . You can use tiresize.com/comparison/ to check the rotational differences. I just did and 195/65 R15 and 225/45 R17 are exactly the same. This is for new tyres, of course. As they wear, rotations per mile become greater and so does the speedometer over-read.
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