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post #8 of (permalink) Old 27th June 2010
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Re: Speedo readings

Speedo's over-read by up to 10%, in fact they have to by law.

Why? So that when you get done for speeding you cannot blame it on the speedometer giving a dodgy reading.

For example when the speedo reads 70mph you are really doing about 65, which is a nice safe margin for your driving licence. If the speedo told you were doing 70 when you were actually doing 75...

This is why your Sat Nav gives you a lower - more accurate - reading.

The speedo calculates your speed based on how many times a second a wheel rotates (usually the front wheel), and the rolling radious of the tyre.
Change the rolling radius - by tyre wear or fitting different tyres - and the maths produces a different answer. This is why you have to be careful with tyre sizes when upgrading from 15" to 17" wheels for example.
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