I think this over read of the speedometer is a common problem. My speedo reads 76mph when the GPS shows 70mph, about 8% error. I put this on my list of faults for the first 1000 mile service thinking it would be an easy laptop job to do a recal, it is only a glorified bicycle computer after all. I was told it was OK as it was less than +10% error. Later when the car was returned to the factory for other issues, I mentioned the large speedo error. They "had a look at it" but the error is still the same. Most of these type of systems have a datum pulse figure that can be altered to reduce the error but with no information available and the dealer or factory unwilling or unable to do anything.......
