David F wrote:

Perhaps I'm being thick Roland but I don't really understand the point
you're making - how is "a true 80mph" in fact "typically [...] over
95mph indicated" ?!

The diference between what the speedomoter reads and the actual speed.
I have a Origin B2 in my car which has GPS speed indication. The
difference between it and my cars speedomoter is roughly 3mpg ...

You appear to be comparing apples with oranges! :-)

Peter Beale