Speedo and satnav show different speeds - which is more accurate?

My Garmin nüvi 1300T satnav consistently shows 4mph less than the speedometer reading on my Qashqai 1.6 Acenta manual, checked on a level road. For example, at 30mph on the speedo the satnav reads 26mph. At 70mph, the satnav reads 66mph. Which should I trust?

Asked on 16 June 2012 by GB, Saffron Walden

Answered by Honest John
In this circumstance your Garmin will be accurate, the car's speedo will over-read and the speed warning sign will over-read. But with speeding revenues down, I'm now hearing of drivers getting done for just a few mph over the limit, despite ACPO guidelines, so take care.
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