How accurate is my SatNav's speedometer?

Father Christmas brought me a satnav for Christmas – woohoo! It’s really good. In addition to stopping me getting lost, it also tells me how fast the car is going. My question is: Is the speed on the satnav screen more or less accurate than my car’s speedo? I guess it should be more, but my speedo appears to be out of true and the faster I go, the worse the discrepancy gets. I ask because where I live, the police are very tricky, particularly at this time of year. The last time I was doing 36 in a 30 limit, its cost me £60 and three points on my licence!