If the cop told you that the radar had not been calibrated, take that to court. It should be good for a walk. To the best of your knowledge, you were not speeding. Your eyes were on the road and the traffic, not the speedo, so you can't be sure, exactly. Dress nice, say yes sir and no sir. This ought to get you if not a complete walk, at least a supervision or whatever they call it in your state.