All but one car speedos I have tested were out but tend to be 2.5 mph fast at all speeds
rather than 10% fast so the gap gets bigger as speed rises, I did have a Mazda Bongo
that was dead on and when you first got in it you did seem to be rushing about a bit,
I also had of all things a Greeves that was dead on but suspect it was more luck than
good judgment.

Interesting...Presumably your Bongo was an import so someone fitted a kph/mph conversion when imported rather than a factory speedo?
.btw it was the Tmax mk4 speedo that was way out, the mk6's seems to only the normal 10% off!