I have a friend, Col, who has been a transport driver for a great many years. He informs me that the speedos in large vehicles are very accurate and calibrated to show actual road speed.
I told you that story to tell you this one.
Col has a GPS. When he drives sedans or small vans the GPS always indicates 3 to 6 klms/hr less than the speedo. However in the large transports the GPS agrees with the speedo.
Sometimes there are devices which indicate vehicle speeds on the roads. In huge lights so the driver can see the read out on approach. The GPS always corresponds with the speed check device.
I hope this places a little more light on the subject
Cheers