Is my Road Pilot GPS system more accurate than my speedometer?

I have recently taken a subscription for Road Pilot, which warns one of speed cameras and shows my speed on my iPhone (which is fixed onto my car windscreen). I notice that typically my car speedometer shows say 73/74mph, while the Road Pilot shows 68/69mph. My query is, which is correct in the eyes of the law/police?

Asked on 28 December 2013 by OS, Southport

Answered by Honest John
If it's GPS, on a flat road under a reasonably clear sky your Road Pilot will generally be accurate and your speedo will generally over-read. ACPO recently released new guidelines on speeding penalties. They are at:
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