@flambonkscious@Dave The guy who services my car told me many moons ago that some vehicles are set to read slightly higher than they really are - as a safety thing.
¯\_(ツ)_/¯
Ooh that's interesting. I have seen this in almost every car I've ever driven past one of those things. There's actually one car that I remember borrowing one time that stands out in my mind as it matched the speed sign thing and that was unusual enough to still remember it years later.
I just assumed that since the speed is read internally (gear box or something?), as the mm if rubber come off the tire the speedo would over read more and more.