I don't think speedos 'have' to over-read .... they are 'allowed' to over-read (and by a big chunk ... 10%+6ish miles an hour .... showing 50 while you're only doing 40!) .... what they mustn't do is under-read .... so car manufacturers tend to err on the side of caution and make them 'fast' ..... not a requirement though (and my Dedender is spot on actual speed) .... what it means is that a driver could have an accurate speedo and get a ticket for drifting to 41 in a 40 limit .... which I think is a bit harsh ... especially as that sort of error could be down to the particular brand of tyres ( even of your usual size) that your fitter chose to put on your car with you being none the wiser
K
