The global position

I have just started using Sat nav for the first time and have noticed that the speed reading on the Sat nav is usually 4mph less than on my speedometer. When driving at the speed limit which should I rely on?

Asked on 19 September 2010 by JF, via email

Answered by Honest John

On a straight, flat road the satnav will be 100% accurate because it uses GPS to measure your speed across the surface of the planet. It is less accurate on hills and twisty roads. Speedos are always set to over-read because by law they are not allowed to under read. Your 4mph over-read is typical.

Tags: sat nav speedometer

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