Speedometers are generally calibrated to three points on the dial. In the case of after market, I think it would be best to set for 70 with a GPS or State trooper.
Mine is set right on by asking a trooper to ride next to me at 70 his car and my GPS. Actually I asked him to drive 70 on I75 and I got next to him. He was a very nice person, one of the guys who came out for the inspection.
