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Speedometer
A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle. Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards. Speedometers for other vehicles have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.
Charles Babbage is credited with creating an early type of a speedometer, which was usually fitted to locomotives.The electric speedometer was invented by the Croatian Josip Belušić in 1888 and was originally called a velocimeter.
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Tachometer
A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common.
The word comes from Greek ταχος (tachos “speed”) and metron (“measure”). Essentially the words tachometer and speedometer have identical meaning: a device that measures speed. It is by arbitrary convention that in the automotive world one is used for engine and the other for vehicle speed. In formal engineering nomenclature, more precise terms are used to distinguish the two.
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Speedometer (noun)
A device that measures, and indicates the current speed of a vehicle.
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Speedometer (noun)
Such a device incorporating an odometer.
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Tachometer (noun)
A device for measuring the revolutions per minute (RPMs) of a revolving shaft, as with the driveshaft of an automobile.
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Tachometer (noun)
A device for measuring or indicating velocity or speed, as of blood, a river, a machine, etc.
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Speedometer (noun)
an instrument on a vehicle’s dashboard indicating its speed.
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Tachometer (noun)
an instrument which measures the working speed of an engine (especially in a road vehicle), typically in revolutions per minute.