Checked against GPS our three cars VW polo GTI, Fiesta Titanium tractor and Plus 8, they all read 5-6 mph high at 70 and 3mph high at the 30 point.
I have always used the speedo to go by and have only had 3 speeding fines over the 53 years of driving my last being in 1991, so I'll stick with my method and ignore the semantics.
Keith's advice is the best tbh he's at the coalface.
