Like, how do they work? They’ve been around since nearly as long as cars have been around so it’s not some fancy modern engineering. How does it know what speed the cars going?
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Modern speedometers are somewhat of a bit of fancy modern engineering. You have a sensor which give a pulse every time a teeth in a cog passes and then there is a microcontroller counting these pulses and calculates the speed which it then use to send signals to the small servo motor in your speedometer dial.
Old speedometers though worked a bit different. There were a cable connected to the output shaft of the transmission all the way to the dial. This cable would then spin with the wheels. In the dial this cable is connected to a magnet which is therefore also spinning. Over this magnet there is a cup, which is eventually connected to the needle. The magnet and the cup does not touch directly but are magnetically coupled so when the magnet moves the cup also moves. However there is a spring pushing the needle back to zero. The faster the magnet moves the more it pushes on the cup and the further the spring is tightened pushing back against the magnet. You can read off the speed from the dial.
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