How does a speedometer work?
A spinning magnet in a speedometer acts with magnetic force that pulls the pointer and causes it to register the auto’s speed. The speedometer in an automobile tells how fast the car is going.
A speedometer operates by magnetic force. Inside a speedometer there is a magnet that is connected directly to the car’s transmission by a flexible cable.
When the car moves, gears inside the transmission turn the cable which spins the magnet.
Fitting over the magnet is a special metal drum called a “speedup.” The speedup is attached to the pointer in the speedometer’s dial.
The spinning magnet acts with magnetic force that pulls the speed cup and it causes the pointer to revolve against the restraint of a spring.
When the car speeds up, the faster the magnet spins and the greater is the force exerted against the spring.
At the magnetic pull gradually overcomes the resistance of the spring, it causes the speedometer to register a higher speed. When the car stops, the spring pulls the pointer back to zero. – Dick Rogers