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Speedo question


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I was the motorway the other day, my speedo was reading 80 but according to the sat nav my speed was 75.

Is one more accurate than the other? and if so is 5mph difference a problem?

Sorry for all the questions just curious.

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Speedos are always out. the manufacturers add 5-10% on to allow for any variation thanks to wear and tear.  If it's out the wrong way they can be heavily fined.

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GPS will be 99.9% correct.

Speedo has to be within 10% of the actual reading but cannot under read, so they always over read to allow for a margin of error in the way it calculates your speed. It doesn't know how much tread is on the tyre, affecting the circumference for example.

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