they give you 10% +1 leaway usually (78mph in a 70, 67 in a 60) but this isnt the law, so they can technically get you for 72.
also, your speedo is never truly asccurate, if you've ever driven with TomTom you'll notice your speed is always around 4mph less than displayed on your dials. fingers crossed you were doing 76 and they'll let you off.