A speedometer is the instrument in a vehicle which shows how fast the vehicle is moving.
speedometer in British English
(spɪˈdɒmɪtə)
noun
a device fitted to a vehicle to measure and display the speed of travel
See also mileometer
speedometer in American English
(spɪˈdɑmətər)
noun
a device, often combined with an odometer, attached to a motor vehicle, etc. to indicate speed, as in miles per hour: it is usually connected to the transmission by a flexible drive shaft