Australia

I misstyped. It's supposed to be younger not older. Older cars are still required to show the correct speed however in 2004 manufacturers started putting in a 'buffer'.

Cars manufactured before 2004 will generally have a very exact speedo that has the potential to display below the travelling speed under certain circumstances, eg bigger wheels than intented for the vehicle, overinflated tires, etc.

After 2004 manufacturer's started putting in a 'buffer' of anywhere between 4 to 12 km/h due to new regulation preventing the previously mentioned situation.

For example a Taraga people mover can be anywhere from 7 to 12km under the displayed speed, Camry hybrids are almost always about 4km/h slower than their shown speed, Prius are mostly 5km/h slower on the dot.

My 2002 Camry is on the money.

Experience: Few years as a taxi driver.

Very old cars were allowed a margin of error on both directions. Newer cars are allowed zero margin for error on reading low but some margin for reading high (i.e., you are guaranteed to actually be going at the speed recorded or lower—you can never be speeding if your speedo says you're under the limit).