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Inside a mechanical speedometer

Inside a mechanical speedometer Drive cable Flexible tube Hair spring Needle Milometer Dial Metal drum Magnet

A flexible drive cable inside a flexible tube links a small magnet inside the speedometer to the gearbox output shaft.

The magnet rotates with the shaft and its magnetic field attracts a metal drum to turn the speedometer needle against the force of a hair spring. The needle moves round the dial until the restraining force of the hair spring brings it to rest to give a reading of the road speed.