A tachometer is an instrument that measures the working speed of an engine, typically in revolutions per minute (RPM). Daniel Reinhardt/dpa/dpa-tmnA tachometer is an instrument that measures the working speed of an engine, typically in revolutions per minute (RPM).You find them in cars and also in boats, planes and other vehicles.Most tachometer gauges have either an analog (dial) or digital (LCD or LED screen) display.Regulations state that a vehicle's speedometer must never display a speed that is too low.Conversely, however, it is permitted to run ahead – in other words, to show a higher speed than the car is actually travelling at, says Germany's TÜV Nord certification authority.AdvertisementAdvertisementDepending on where you live, the rules differ. For vehicles registered before 1991, speedometers may deviate by up to 7% from the actual speed, in accordance with EU Directive 75/443/EEC.For vehicles registered after 1991, the speedometer needle or digital display may even show up to 10% more – plus an additional 4 kilometres per hour.That means that the law, depending on where you are, permits a speedometer reading of up to 114 km/h if the car is actually travelling at 100 km/h.Check accuracy with sat-navAlthough carmakers do not usually make full use of this tolerance, the speedometer generally displays a slightly higher speed, according to testers.AdvertisementAdvertisementYou can easily determine this difference by using a sat-nav. GPS satellite navigation shows the discrepancy on the display is only 1 kilometre per hour, says TÜV Nord.Watch your speedDrivers should keep a close eye on their speedometer or risk being fined or face further penalties. If you are unsure how fast you should be driving, be careful to check."Without sustained, 100% concentration, it is incredibly easy to miss developing hazards on the road. Distractions or a wandering mind drastically reduce your reaction time, making even familiar routes dangerous," notes Britain's Automobile Association.AdvertisementAdvertisementBear in mind that however much you may be in a rush, it is not safe to speed, as you and others face potential consequences if you speed and crash.Try and recognise and resist pressure to speed from yourself and others, say automotive organizations, to improve safety on the roads.