This is more for a broader information for those that are interested.
I am a process instrumentation technician by trade. So calibration, accuracy and repeatability are all very important terms in my field.
Speedometers, are often not very accurate or deliberately set to over read by a certain amount. However, they are very repeatable. In other words, they will always show the same error every time. In our trade, this is often more important than absolute accuracy, particularly as accuracy can be corrected, repeatability often can not as it is usually the indication of a worn part or poor manufacture.
Speedometers will gradually over read as the tyre diameter slowly decreases with wear, but most would not even notice this.
It's much like the grouping on your target at the shooting range. A wide grouping shows issues with technique, rather than a sighting problem, where a small group off the bull is much preferred as a small adjustment on the sighting will remedy this. Usually a single point correction (with the Rocket this is a field in TuneECU that can be set) and your speedo will be close. Having a rotating needle type speedometer (opposed to a digital readout) an additional user read error (error of parallax) is also introduced. In high end calibration gauges, they had a mirror strip behind to overcome this. Me, at 6'6", would read the speedometer slightly differently than say someone who was 5'.