Sometimes while driving the speedometer will show 40 mph when driving 30mph?
speedometer fluxuration on 1993 Acura Vigor
It's funny I verify speedometer operation on customer's cars using my Garmin GPS system its pretty accurate. Usually speedometers read slightly over the speed you are doing. A professional grade scan tool reads live sensor data which would verify the operation of the engine/transmission Vehicle Speed Sensor (VSS) but the VSS is an a/c signal generator it would fail at all speeds. That would leave the speedometer head as being the problem.