Speedometers, as with any gague, is most accurate in the center third of its range. I, too have noticed the 2 mph difference with the speedometer and traffic gun (mine is also 2 higher.0 in the 45 to 70 mph range. Had my brother the cop also check me with his Radar, same result. I couldn't pin him down on the police radar's accurancy (trade secret I guess, like the handshake to avoid tickets) but he says that's why they don't ticket anyone for 2 mph over the limit.
300A
born 08-27-02
Adjusted air in and air out.
400+ timeslips.
When in doubt, GAS IT! This will either fix the problem, or end the suspense.