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Is my satnav more accurate at reading speed than my speedometer?

I notice in most cars a satnav always seems to indicate a vehicle speed 2 or 3 mph less than the car speedo. I assume that the satnav should be absolutely accurate, but would appreciate your comments.

Asked on 17 March 2012 by GH, Caterham

Answered by Honest John
On a flat straight road under a clear sky with the satnav picking up three or more satellites, it will be 100 per cent accurate. But on hills, twisty roads, in towns with tall buildings or in tunnels, it will simply guesstimate your speed.
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