How Does A Speedometer Work?


Have you ever been cruising down the road and pass a police officer? The first thing you are likely to do is slow down, regardless of how fast you were going, the next is to check your speed using the speedometer on your vehicle. Have you ever stopped to think about how some of the technology in our lives works though? I’m amazed to be typing on my laptop and sending up blog posts to space which end up on your google browser. How crazy is that?

Speedometers are less high tech than you might think. They simply measure how fast your wheels are turning and does some math regarding how big your wheels are and calculates out how fast you are going. Police officers use radar to measure your speed which involves a laser beam that bounces off of your car and back to its source, thus telling the machine how fast you are moving. It’s obviously more complicated than that, but that is the gist of it.

The Types Of Speedometers

Did you know that there is more than one type of speedometer? They accomplish the same things but do so in different ways.

  • Mechanical Speedometers
  • Electronic Speedometers

Mechanical Speedometers

As you press your gas pedal, the wheels of your car spin faster or slower according to how much gas is being supplied to the engine cylinders. When you want to check how fast you are going, you check your speedometer, which tells you the speed in kilometers and or miles per hour.

The speedometer gets it’s speed reading by gauging how quickly the wheels are spinning it then uses electromagnetism to convert the spinning wheel energy into a smooth drifting motion in the gauge. The cable will measure the rotation and feed back to the speedometer turning a magnet.

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.

The Specifics If You Care

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.

Electric Speedometers

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.

How Electronic Speedometers Work

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. The speed is then displayed on the electronic LCD display. Not only does this system count speed, it also keeps track of mileage and feeds data to the odometer.

What Does A Speedometer Measure?

The speedometer is the device in a vehicle that captures the speed and displays that data to the driver through the dashboard of the vehicle. There are several types of speedometers that work in various ways. They all measure how quickly the wheels rotate. Another component similar to the speedometer is the odometer. The odometer captures how far the vehicle has traveled.

How To Read A Speedometer

The speedometer is part of the instrument cluster on your vehicle which displays important vehicle statistics such as speed, miles traveled, temperature of the vehicle, and fuel levels. The instrument cluster also is where you will find warning lights for your car.

The speedometer has two separate semicircles which contain a set of numbers. The smaller semi circle shows numbers in kilometers and the exterior semi circle typically displays figures in miles. As the hand of the speedometer sweeps through the circles, where it sits is the current speed of the vehicle.

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