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How do speedometers work?

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Updated 27 Aug 2019

Joel Ilton

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There’s nothing worse than travelling on the highway, cruising along listening to your favourite music, when a van on the side of the road catches your attention - a speed camera!

Looking down, you see your speedo is showing 115km/h and there’s no time to slow down but as you fly past, there’s no flash.

Did you just get away with it or are there other things at play here? This is where knowledge of how the speedometer system works comes in.

The first speedometers fitted to vehicles were operated by a cable.

This cable was either fitted to the gearbox or down near the wheels and was directly connected to the back of the instrument cluster.

Using electromagnetism (that’s the big word of the day) the cable would spin faster or slower depending on the wheel speed.

The speedo needle was connected to a magnet, and the spinning of the cable would create a magnetic field, which operated the needle.

Going faster would increase the magnetic field, which would move the needle upwards, and slowing down would move the needle back down again.

Seems complicated, right? This is actually the simpler system, as modern day speedometer operation is much more complex.

The cable system began to be phased out in the 1980’s as more vehicles were fitted with anti-lock braking (ABS) and wheel speed sensors.

These sensors allow for a much more accurate speed reading as they monitor wheel speed in real time and send a digital signal to the engine control unit (ECU).

This signal is then passed on to the instrument cluster.

Sounds simple, but let me explain in a bit more detail.

This system also uses magnets, which are either fitted to the driveshafts or wheel bearings.

These magnets spin at different speeds as the vehicle moves, and special sensors, called hall sensors, pick up this movement as an electrical pulse.

This pulse is sent up to the ECU through a wiring harness, which then calculates the speed using parameters set by the manufacturer.

This speed is then shown on either an analogue speedo (using a needle to indicate speed) or, on more modern cars, via a complete digital readout.

This system keeps track of wheel rotation as well, so also provides the odometer reading for your vehicle. Put simply - wheels turn, send signal to computer, displays speed.

You might be thinking to yourself, ‘that hasn’t answered why I didn’t get speeding ticket’ - and you’d be quite right.

To explain that, we first had to go through how the speedo works. Now that’s out of the way, we can dive into speedo accuracy.

From 2006 onwards, the Australian Design Rules (ADRs for short) dictate that all new vehicles must have a certain amount of leeway when it comes to displaying speed.

Most new vehicles will read between 5km/h and 8km/h over your actual speed, so if you are travelling at an indicated 100km/h, it's possible you are only doing 92km/h!

If you want to read more about this, the Australian Government has released the official document, which can be seen here. Fair warning - it may put you to sleep before you get to the end of it!

Understanding the fundamentals of how your speedometer works is key to make sure you are operating your vehicle inside the limits of the law, and the next time a friendly police officer asks you how fast you were going, you will understand exactly how your car showed you when you answer.

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Written By

Joel Ilton

Finding a passion for cars from a young age, Joel carried out work experience as a mechanic whilst at school before starting an apprenticeship after finishing year 12.

After almost 10 years on the tools and in customer service, he moved into the IT realm as a Data Analyst and In-House mechanic at AutoGuru.