The European formula
Now it gets really crazy. The European Union requires adherence to UN ECE Regulation 39. It’s a lot of math, but the simple version is that no speedometer can read slower than the actual speed. Ever. On the high side, it’s allowed to read up to 10% above the actual speed plus four or six kilometers per hour, depending on the type of vehicle.
Compare this chart to the first one. The most a speedometer can be off at that same 100 mph (160 kph) is nearly 13 mph. This is why many BMW and Porsche drivers notice their speedometer is off.