I'm not going to quote the long post by fellow Rver from PA...but... no speedometer ss accurate to one mph except at one point in the wear of the tires perhaps.
There is no Judge unless he is running a ticket factory going to fne your for one mph over. He does appeal, though costly will get it thrown out and maybe that judge counciled.
here in Tn and in parts of Pa I have been in a car on cruise and especially a Mh, or your foot on throttle and brake cannot keep speed on the mark! Our little AMG with tremendous torque engine braking might in fifth gear but not 6th or 7th but I doubt it becau of the hills and even minor rises and dips.. They tried that trick on downgrade of Interstate 81 I think, north of Knoxville years ago, and in many places around the Country until the State got involved.And in VA years ago using it as excuse for drug checks or something and the money.
State put an end to that too. Around here 10% or 10 mph on the interstates or more. Keeping with the flos of the traffic as is the wise logical and safe thing to do.