It is completely true. I have never driven a car that does not do this.
If you have a GPS, compare the GPS speed to the speedometer.
Your GPS should be 99.99% accurately describing your speed, unless it is a dud.
The speedometer will most likely be over reporting by 10%.
The law requires speedometers to be accurate to +/- 10%. Most manufacturers err on the side of caution.
The old bill can't book you if you are within this speed range. eg 77mph is just ok on a dual carriageway.
Changing wheel and tyre size will effect your gearing and the speedometer reading.