My VW Golf overstates by around 7 kms per hour. From various forums this appears to be a feature, not a defect, of German vehicles.
My previous 2007 Passat also had this 'feature'. Having driven VWs last 7 years, I've started to appreciate this as my overstated 'speeding' was given a leeway margin of 10% or so, especially when driving in places like Guelph where police don't really have much to do except sit there with a radar gun.