according those flashing speed signs you see about the place my cars doing 30mph when my speedo reads 33mph
My sat nav confirms the reading when doing 70 on the sat nav reads 76/77 on the speedo
Construction and use regulations say speedos must not underread, but can overead by up to 10%. The more accurate a speedo is, the more its costs to make so its no surprise to see the GPs speedo is bang on the 10% limit.