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What's more accurate - a speedo or a satnav?

The speed recorded on my satnav is 3mph slower than the speedometer reading, which is likely to be accurate?

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The satnav shows your GPS speed across the surface of the planet triangulated by at least three satellites. So if you are on a flat, straight road, the speed it shows will be your true speed. Your speedometer will be out.
Tags: speedometer
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