Incorrect car speed shown
So, I'm driving say on a motorway, perhaps travelling at 70Mph. The sat nav is working fine, I then slow down to say 50Mph and the sat nav still shows 70Mph, and stays showing 70 for a few minutes and then it shows the correct speed again. Then it seems to work fine for a while, updating the speed as the car changes its speed.
Any ideas why this occurs?