Didn't most companies factor in 10%, so you couldn't blame the speedo if you got caught speeding? I think legally they now have to make it accurate... although I have no source / evidence to back this up!
My SatNav (on my phone) is always set to show the gps speed, and ive noticed on all my cars the speedo speed is usually around 6->8% over. For example, 70mph on the sat nav is 76mph on the dash.
That's why when I changed my dials I set my speedo to the nav