Quote Originally Posted by brad View Post
That would be why you'd double check the true speed using at least one GPS rather than just taking blind advice from a forum.

All our cars are different & the diameter of our tyres can vary markedly from brand-to -brand even if they are the same nominal size.

Martinnw seems to be at the extreme end of innacuracy.
I was going to say the same thing.
Seems like a 10% speedo overreading.