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How Do Speedometers Work?

CarPart  ·  January 25, 2023

How Do Speedometers Work?

If there’s one thing that you look at and check on your car’s dashboard every few minutes, it’s the speedometer. Ironically, it’s the most overlooked item when it comes down to understanding how it works and how it might fail. 

Speedometers have mechanical or electronic components with corresponding digital and analogue displays. Regardless of the type, they connect to your vehicle's transmission to calculate how fast the car moves. Mechanical speedometers rely on cables, springs, and magnets to turn the display dial, while electronic types use sensors that send data to the car's onboard computer.

You’ll learn everything you need to know about speedometers in this guide. First, you’ll discover the different types before understanding how they work. Lastly, this guide will also show a few ways the speedometer might fail.

Let’s get started.

What Kind of Speedometers Are There?

Before diving into how speedometers work, one must understand that two different kinds exist. Not only is there a difference in the types of speedometer components, but even their displays can differ depending on the vehicle brand and model.

Mechanical Vs Electronic Speedometer Components

Firstly, speedometer components come in mechanical and electronic versions. As you can imagine, mechanical speedometers use several moving parts to measure how fast your car goes. Meanwhile, electronic ones rely on sensors to collect the same data before communicating it to you.

Late-model vehicles rarely include mechanical speedometer components, as most have switched to electronic ones instead.

However, that’s not always the case regarding speedometer displays.

Analog Vs Digital Speedometer Displays

Even with cars using electronic speedometers, many models still rely on analogue or mechanical displays. Those are the ones with a needle that moves up and down, pointing at numbers representing your car’s current speed.

In other words, electronic speedometer components are compatible with analogue displays.

So, while mechanical speedometer components have essentially been phased out over the years, analogue displays have not.

No matter the type of components or display your car’s speedometer has, what you need to know is that it provides you with critical information. 

The speedometer helps you gauge how fast you’re going so you can stay within safe speeds, avoiding fines and maintaining optimal control over your vehicle.

How Do Speedometers Work?

Now, let’s dive a little deeper into how speedometers work.

How Do Mechanical Speedometers Work?

Mechanical speedometers are connected directly to the transmission’s output shaft. That’s because the output shaft offers the most accurate measure of how fast the car moves on the road at any given time, making it the most reliable source of speed information.

The speedometer display is directly linked to the chive pinion inside the transmission. When the transmission moves faster as the car accelerates, the chive pinion causes the display needle to turn, pointing at the corresponding number on display.

Meanwhile, that display needle is connected to a spring that will always return the needle to 0 (zero) on the display. The faster the car goes, the more the pointer will turn; when the vehicle slows down, the spring returns it to its original position.

How Do Electronic Speedometers Work?

As you can imagine, electronic speedometers don’t have as many moving parts as mechanical types, but they have more sensors and wiring.

Electronic speedometers also connect directly to the vehicle’s transmission. Instead of a cable or gear, it uses a sensor to measure how fast the gears turn.

The sensor gathers split-second data of how fast those gears turn and communicates it to the car’s onboard computer. The computer then calculates the vehicle’s speed based on that data and displays it on the digital or analogue display.

The entire process described above happens continuously, so you’ll always have accurate information on your dashboard or instrument cluster about how fast or slow you are driving at any given time.

Speedometer Vs Odometer: What’s The Difference?

The speedometer is often placed alongside the odometer, causing many people to confuse the two. While they rely on the same data, the odometer is only there to measure the distance your vehicle has travelled.

Odometers also come in analogue and digital forms. Not only do they measure the lifetime mileage of your car, but you can also use them to measure the distance of a particular trip.

That makes it quite useful if you need to measure how far a destination is and calculate the potential fuel costs involved.

The information from the odometer is crucial for other reasons, including planning when you should bring your car in for routine maintenance. It also helps prospective buyers gauge the wear and tear of the vehicle they want to buy based on how many kilometres it's been driven.

What Causes Speedometer Problems?

Mechanical and electronic speedometers rarely fail, as both are designed to be highly reliable, yet problems can still happen, such as:

The speedometer is a critical component in your car, so you must get it fixed as soon as possible when you notice symptoms of a problem.

To learn more about car parts like speedometers and other essential components, stay tuned to the CarpartAU blog. It’s updated frequently with articles teaching you about your car and how you can solve problems that you can easily detect or fix.


By Ray Hasbollah

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