After my IAM training I spent a while self calibrating my speedo from GPS and found with my standard tyres at the recommended pressure an indicated 33mph was 30, but an indicated 68mph was 60! Gives a better realisation of exactly what speed you were doing, if for some reason you had to discuss this fact with a uniformed officer...ahem

Actually met one of my closest police friends in this sort of situation. I was roundabout playing and he spotted me, but after discussion he realised I was exactly on the speed limit and although drifting trough each roundabout, I was staying in my lane and obviously in control. Turns out he was an IAM examiner and went to some of the same track days and a general good guy.