Misread the slow reading in the original post. My speedometer reads 60 mph and the gps shows 65 mph, for example, so I am always travelling more quickly than the speedometer is indicating. I would be happy with 10% over but 10 % under puts me at risk of a speeding fine even when I am trying to behave.
Same point though in that if I find out how to adjust the speedometer I will share the outcome.
