....that was on the flat on the MTB on knobblies!
....sadly it's not true. 3 or 4 times during the 5 years the Cateye has been on the MTB I've had spurious readings and every time I can recollect a (relatively) close pass by a Police car or ambulance on flashing blue lights and siren. So I'm putting it down to radio interference on the wireless transmitter.
A more accurate figure would be 30mph (on the flat on knobblies, with a tail-wind).
One bloke I know was stopped and cautioned by the Police for doing 37mph in a 30 zone.