What A meter fixed to a vehicle that measures and displays its speed?

Answer 1

In most vehicles it is a speedometer; in commercial vehicles a tachograph is used.

The tachograph not only measures and displays the speed but also records the distance and speed driven over time; it is also more accurate than those in cars with an error of about 1-2% whereas those in cars are only accurate to about 10% and generally over estimate speed. They are checked every 2 years for calibration and recalibrated every 6 years unlike those in cars.