Speedometer: Digital or analogue indicator placed on the dashboard that shows the speed of the car.
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An iron cable or an electrical sensor monitors the rotation speed of one of the wheels (usually the front one) and, through a mechanism or certain mathematical calculations that take place in the ECU, the moving speed of the car is shown in the speedometer, which may be analogue or digital.
When I turn on my car the speedometer shakes at zero and my revs sit at 0 and if I drive like this every now and then the speedometer will hit 220 kmph