Your speedo is more than likely optimistic, usually by about 10% or so, but dont count on it. Usually, but not guaranteed, you need to be doing 11 miles an hour over the limit before getting busted, if I presume the speed limit was 70mph, you may well get away with it, if however it is 30, 40, 50 or 60, you will get a call from an officer accompanied by an onsite car crushing lorry, which is a good laugh for friends family and neighbours.