What is a GPS speedometer and is it better than my regular speedometer?

Recently, there has been an influx of products known as GPS (Global Positioning System) speedometers popping up online.

These are similar devices to what is in portable satellite navigation units and the claim is they are more accurate than the speedo fitted to your vehicle by the manufacturer.

Sound too good to be true? Well, first we need to look into the operation of the speedometer.

The speedometer in your vehicle is designed to show your current speed at all times.

Modern vehicles use the ABS (Anti-Lock Braking) or vehicle speed sensors to determine the rotational speed of all four wheels.

This information is then transmitted by a wiring harness up to the ECU (Engine Control Unit) which crunches the numbers against the manufacturers predetermined figures.

This calculates the speed of the vehicle, and is shown on the analog or digital speedometer.

While this system is reasonably accurate - worn or underinflated tyres, aftermarket wheels or even different brands of tyres can throw this reading off.

There is also a margin of safety programmed in from the factory.

Australian Design Rule (ADR for short) 18, which came into legislation in 2006, stipulates that the speedometer cannot indicate a speed less than the vehicle’s true speed, or a speed greater than the vehicle’s true speed by an amount of 10% plus 4 km/h.

This is typical politician speak, and anymore of it is likely to put you into a coma, so let’s talk about it in human terms.

A simple example would be your vehicle travelling at 100km/h.

This is the vehicle’s ‘true speed’, as mentioned above.

Using this ADR, the manufacturer could program the speedometer in your car to indicate anywhere between 100km/h (true speed) up to 114km/h (10 percent + 4km/h)

Super confusing, right? This is mainly done to limit any issues that could arise from inaccurate speedometers reading below the vehicles true speed - a ‘better safe than sorry’ clause for the manufacturer.

For vehicles that were built before 2006, the speedometer accuracy could be +/- 10 percent of the vehicles true speed.

Using the same 100km/h true speed from above, your vehicle could be travelling anywhere between 90km/ and 110km/h, which may lead to an unexpected speeding fine!

This is where the GPS speedometer comes in.

The speedometer in your satellite navigation or dedicated GPS speedometer communicates with the 30 or so satellites that orbit the Earth at certain time intervals, and carries out a calculation to determine how fast you are travelling.

This is great if you are travelling along a straight piece of road with no overhead interference, as the satellites can communicate easily, thus providing you with a very accurate speed reading.

However, if you enter a tunnel or start driving on a windy or hilly road, the accuracy can fall as the signal has to ‘fight’ its way through.

So, which speedometer is better?

This is a difficult question to answer, as it depends on your driving conditions, vehicle age and mileage and if any modifications have been fitted.

If you really want to be safe, it’s best to rely on your factory fitted speedometer, but having a backup GPS speedometer so you can compare the two is always a good option, especially on vehicles that were built before 2006.