How do speedometers work

On the end of this shaft is a magnet. Positioned close to but not touching the magnet is a cup-shaped metal drum that is attached to the needle giving the reading on the dial. A small coiled hair spring holds the needle at zero. The drum is attracted by the magnet so, as the magnet turns, the drum turns too. The faster the car is travelling, the greater the pull of the magnet on the metal drum and the further the needle moves round the dial. But the restraining force of the hair spring also increases as the needle moves round the dial.

At a certain point the forces of the spring and the magnet balance out and the needle steadies to give a reading. The two other common types of mechanical speedometer give the reading by a bar or a mark moving along a straight calibrated scale. Both are roughly similar in operation to the round dial type of speedometer - a cable driven by the gearbox output shaft turns a magnet which causes some sort of indicator to move against the force of a restraining hair spring. In one type the indicator consists of a moving ribbon attached at each end to a spool.

The magnet causes the ribbon to reel off one spool to the other against the force of the hair spring. As the ribbon moves, a mark on it lines up with the calibrated scale to give a reading. The other type of indicator using a straight scale rather than a dial has a barrel marked with a line.

The magnet causes the barrel to rotate until stopped by the hair spring. That magnet sits inside something called a drag cup, which is made of a non-magnetic metal, usually aluminum.

The magnet spins as the wire rotates, creating a magnetic field that forces the drag cup to turn in the same direction. Attached to the drag cup is a spindle that connects to the pointer on your speedometer. A spiral spring on the spindle serves to counter the rotation of the drag cup. If you accelerate, the increased magnetic field moves the drag cup further around; as you slow down, the spring tugs the pointer back. The whole system is calibrated so that the pointer indicates the proper speed.

When you stop moving altogether, the spring pulls the pointer all the way to zero. Digital speedometers are a little different. A car with a digital speedometer uses a speed sensor, which usually consists of a magnet surrounded by a wire coil, quite like the pickup on an electric guitar. The sensor is mounted directly next to a gear on the transmission, and as the gear spins, its teeth whiz by, interrupting the magnetic field on the sensor.

The process is similar to the way a vibrating string activates a guitar pickup. The movement of the gear produces an electrical current that is directly proportional to the rotation speed of the gear. A computer chip processes the current and translates it into a number that corresponds to the speed at which you are traveling, in either miles or kilometers per hour. Got it? The magnet is able to rotate freely within the speed cup which also rotates.

The speed cup is connected to a wire coil and to the gauge itself and dictates the speed reading. So the speedometer cable attaches to the drive train.

As it spins freely with the transmission, it turns a magnet at the same speed. The spinning magnet is located within the speed cup and within it creates a magnetic field that varies with the speed of the vehicle and with the cable turning.

What happens from here is that the speed cup generates electricity. Electricity that really has no where to go. This is where the mechanical speedometer gets its nickname Eddy Current Speedometer.

These rotating electrical currents will cause the speed cup to rotate, trying to match the speed of the rotating magnet. The fine coil prevents the speed cup from fully rotating which causes it to pull the pointer up the speedometer display. The faster the car goes, the faster this entire reaction happens beginning with the spinning cable.

Electronic speedometers are less common but have their benefits. Mechanical speedometers though being seen far more frequently have their draw backs such as inaccuracy and mechanical failure. Electronic speedometers work differently and will have their own benefits and draw backs. Electronic speedometers are newer technology that are quickly replacing their mechanical counterparts.

Electronic speedometers work quite differently from mechanical speedometers. With an electronic speedometer, magnets are attached to the rotating drive shaft and pass by magnetic sensors measuring how quickly they spin. When the magnet passes the sensor, the create a small electric current which allows the sensor to count how quickly the current is recorded.

The sensor and computer then converts this frequency into speed using math.