Is my satellite navigation system more accurate at calculating speed than my speedometer?

I have noticed that my TomTom satnav always seems to show a speed two to three mph less than that registered by the cars speedometer. I use it in two different cars and the difference is approximately the same in each. I have heard that car speedometers are set to read slightly fast, which this might seem to support, but if that is the case what are speed cameras set at and how would you stand legally if you tried to use the satellite navigation as the more accurate instrument?

Asked on 29 June 2010 by IDAPAUL

Answered by David Ross
Manufacturers always set their speedometers a few mph above what is the actual speed, so that you never overestimate your speed. Safety cameras and hand held devices are accurate in a similar way to your GPS but using that to try and get off a speeding ticket wouldn't wash. Plus 2-3mph is unlikely to make much of a difference - if you're doing 32mph rather than 30mph, the police won't prosecute you.
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