17.9 C
New York
Sunday, May 19, 2024

How Does A Speedometer Measure A Vehicle Speed?

A speedometer is one of the standard instruments you can find on a modern car dashboard and often comes with a built-in odometer. Besides being a legal requirement in cars, the speedometer is convenient to gauge your speed while driving, making sure you are not overspeeding and keeping an eye on how many kilometres you have covered.

The odometer is also a good way of checking the mileage of the car. If you want to figure out the best mileage car in India, you will probably make use of the odometer to verify those claims.

With developments in car technology, we are seeing a trend towards electronic speedometers. However, mechanical speedometers are most commonly used in cars because of their simple construction of a needle and calibrated dial. This type of speedometer has been in use for decades and is a reliable method of measuring car speed.

Mechanical speedometer operation

Mechanical speedometers measure the speed of the vehicle as they are linked to the gearbox output shaft. The gearbox output shaft gives the true speed of the vehicle irrespective of which gear you are in.

The output shaft has a small gear that rotates the speedometer gear and cable in turn. The other end of the cable is a magnet that rotates with the cable. Placed close to the magnet is a cup-shaped metal drum that is attached to the speedometer needle. There is a return spring (coil-shaped) that returns the needle to zero position when there is no deflection.

The angle of deflection of the metal cup depends on the rotation speed of the magnet. The greater the speed of rotation, the further the needle is deflected round the dial indicating a higher vehicle speed.

Electronic speedometer

Although mechanical speedometers are more reliable, electronic speedometers are also quite popular in premium cars that make use of advanced technology.

These types of speedometers feature a magnet attached to the gearbox output shaft and an electronic pickup unit placed close to it. As the magnet rotates, the electronic pickup sends a pulse of current through a copper wire to the speedometer unit. Electronics inside the speedometer unit converts these electric pulses into a speed reading.


The speedometer measures the speed of the vehicle depending on the rotation speed of the gearbox output shaft. Hence, it is a simple way of measuring the distance travelled based on the same components.

The odometer is often built into the speedometer because they use the same mechanical components to work. It is driven by gears linked to a cable attached to the gearbox output shaft. The gears drive another series of gears that rotate barrels numbered from 0 to 9.

Some cars have two odometers connected to the speedometer. One measures the distance covered during a trip and can be reset to zero via a dashboard-mounted control. The trip meter is often used to measure the fuel economy of the car.

The other odometer cannot be reset to zero and measures the distance travelled by the car during its lifetime. It is usually marked till 100,000 km and starts from zero thereafter.

Are speedometers accurate?

Speedometers are not 100% accurate as they have a margin of error depending on the state of the mechanical components, road conditions and tyre condition. If the tyre pressure is low or a different size tyre is used, the true road speed and the measured speed will differ by a certain percentage.

Road laws state that the speedometer must be accurate within a range of speeds for safety purposes. It is important to follow speed limit rules, especially when driving through residential areas or where schools are situated.

Modern speedometers are fairly accurate and provide the speed reading without a high percentage of error.


The dashboard instrument cluster contains a variety of sensors, dials and gauges to help you drive your car. Some of these instruments include an oil pressure gauge, coolant temperature reading, fuel level, and engine speed indicator (rpm) and one of the most important instruments, that is the speedometer.

The speedometer is the instrument we rely on most of the time because it ensures that we are driving at the right speed on a given road. The speedometer also has an odometer that measures the distance travelled which can be beneficial when we want to measure how far we have driven on a trip.

With modern technology, we have seen electronic and digital speedometers being used in cars but most vehicles still rely on trusty mechanical speedometers to measure car speed. Today cars are even equipped with heads-up displays where a speedometer is projected onto the windshield so that the driver does not have to constantly look down to find out the vehicle’s speed.

If you found this article on speedometers an interesting read, then maybe you would like to read about the best mileage car in India.

Also Read: Are You Looking for Custom Car Repair in Birmingham?

Related Articles

Stay Connected


Latest Articles