How Does a Speedometer Work?

There are different types of speedometers, like the old mechanical speedometer and the current electronic speedometer. Both of these have a different method to calculate speed.

Speed measurement is performed in your transmission, because that’s where gears rotate at specific speeds and specific intervals according to the speed of your car. From the time speedometers first appeared in cars, designers realized that if they could tell how many times a specific gear rotated and calculate how that corresponded to the speed of the car, they could create a speed meter to measure how fast the car was going. Over the years, these devices have gotten more elaborate and electronic, but the transmission measuring concept remains the same.

Different Types of Speedometers

Here are some different types of speedometers.


Mechanical Speedometer – The mechanical speed-o-meter used a cable attached to a gear found in the transmission. When the gear rotated 1,000 times (usually), the meter on your dashboard was determined by a magnet. This worked because the revolving magnet rotated around a non-magnetic speed “cup”, which produced a magnetic field. The magnetic field manipulated a pointer, which would point at the correct speed on your speedometer.

Digital Speedometer – The digital speedometer that is so common today works with the use of a microprocessor. There is a speed sensor in your transmission which keeps track of your speed. This sensor transmits a signal to the speedometer’s microprocessor. The computer memory would send a signal to a circuit in the electronic display and this displays the digital numbers which correspond with the speed you are traveling.

Quartz Speedometer – Another type of speedometer is the quartz crystal speed counter. A magnet in your transmission generates a signal which is processed by an electronic circuit and sent to either a quartz speedometer or a digital speedometer in your car’s dash.

How Accurate Are Speedometers?

How accurate are speedometers? Well in fact peedometers are not always entirely accurate. In fact, speedometers have a deviation of up to 10% of the speed measured. That’s for a number of reasons, particularly tied to the diameter variations in your car’s tires. If your car’s tires have low pressure or they are not adhering to standards due to the size of the load you are carrying or because of the temperature of the day (expanding or contracting the size of the tire), then the number of times the tires go around may not correspond to the transmission gear which is being tested. This can throw off speedometer calculations easily.

Therefore, I wouldn’t recommend that you simply assume your speedometer is always right. If you’re in a speed zone, don’t push your car’s speed up to the limit of what you think is acceptable for local law enforcement, because tire pressure or some other factor might push your speed limit over the local accepted speeds.