Navigating the roads safely and legally requires an accurate understanding of your speed, but have you ever wondered if your car's speedometer is telling you the truth? With the advent of GPS technology and its integration into our daily drives through apps like Google Maps and Waze, we now have a second source of speed data. But which one should you trust?

The question of accuracy between a car's speedometer and a GPS device is not just a matter of curiosity; it can have real implications, especially when it comes to avoiding speeding fines. Let's delve into the mechanics and technology behind these speed-measuring tools to uncover which is more reliable.

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Understanding Your Car's Speedometer

Your car's speedometer is a carefully calibrated instrument designed to keep you within legal speed limits and out of trouble. However, it's not without its quirks. According to the Australian Design Rules, vehicles manufactured after July 1st, 2006, must have speedometers that do not show a speed lower than the actual speed. This means your speedometer can legally display a speed up to 10% higher than your true velocity, but never lower.

Users of the navigation app Waze compared the usage of traditional car speedometers and GPS-based speedometers. Image Credit: Pexels/Malte Luk

Manufacturers often set the calibration slightly above the real speed to ensure compliance. This is why you might notice your speedometer reading a bit higher than you expect. It's a safety buffer to prevent accidental speeding.

However, this buffer can be affected by factors such as tyre size and wear. For instance, changing to a tyre with a smaller rolling diameter than factory specifications will cause your speedometer to read faster than your actual speed. Conversely, larger tyres will make the speedometer read slower. This is particularly important for 4x4 enthusiasts who might upgrade their tyres for off-road performance without considering the impact on speedometer accuracy.

The GPS Speedometer: A Modern Alternative

GPS speedometers, on the other hand, are not bound by the same regulations as car speedometers. They aim to provide the most accurate speed reading possible by calculating your position changes over time via satellite signals. This can be incredibly precise on flat, straight roads.

However, GPS is not infallible. It can suffer from lag, inaccuracies on slopes or winding roads, and signal loss in areas like tunnels or satellite dead zones. These limitations mean that while GPS can be a useful tool, it's not always the most reliable source of speed data.

The Verdict: Which Is More Accurate?

The accuracy of your car's speedometer versus a GPS depends on the conditions of your travel. A study by the University of New South Wales in 2023 found that while GPS is generally accurate on flat, straight roads, it can underestimate speed on inclines and declines due to changes in elevation relative to the satellites.

Emeritus Professor Michael Regan from the University of New South Wales Road Safety explains that while GPS devices could theoretically adjust for road steepness, the situation is rare enough that most don't. Therefore, drivers should be aware of this limitation when navigating steep terrain.

Can GPS Evidence Overturn a Speeding Fine?

Interestingly, some modern dash cams come equipped with GPS that records your speed alongside video footage. This could potentially be used to contest a speeding fine, especially if the fine is based on an estimated speed by a police officer. While police radar guns are calibrated to legislative requirements, there's a slim chance that GPS data could support your case if you believe the fine was unjust.

However, it's important to note that contesting a speeding fine with GPS evidence is not straightforward. The accuracy of the GPS at the time of the alleged offence, the calibration of the police speed detection equipment, and the conditions under which the speed was measured all play a role in the outcome.

Key Takeaways

• GPS speedometers can be highly accurate, especially on flat, straight roads, but may struggle on slopes or curves and in places with poor satellite reception.
• Car speedometers tend to overestimate speed due to manufacturers calibrating them to err on the side of caution, adhering to Australian Design Rules.
• It's unlikely that GPS evidence will be accepted to contest speeding fines in court, as police speed detection devices are calibrated to legislative requirements.
• Environmental factors like changing your vehicle's tyre size can affect the accuracy of your car's speedometer reading.

In conclusion, while your car's speedometer might not always show your exact speed, it's generally a safer bet for avoiding fines than relying solely on a GPS speedometer. It's designed with a margin that helps keep you within the law. For the most accurate reading, ensure your tyres are the correct size and in good condition, and remember that GPS can be a helpful guide but is not without its limitations. When in doubt, err on the side of caution and follow the speed limits to ensure a safe and ticket-free journey.

 

[Ringdragon]

If one does not have a GPS, then one can only rely on one's speedo. One would assume it is accurate. Getting your speedo calibrated would remove all doubt, or finding out how far out of cal your speedo is. However it is unlikely you would beat a speed camera or radar device if the Police said you were speeding.

 

[CliffE]

The vehicles instrument speed is anywhere 4 to 10 k's under the true (GPS) speed you are doing. My Navara only changes gear above the instrument speed eg. gear change at 43k, 62k, etc. My GPS speedo indicates gear change at 39k, 50/60k, etc. I have had my GPS speed checked by a police radar (he was kind enough to do it at the end of stint checking speeds). My GPS speedo has the facility to adjust the speed up or down and I have it set at 2k's slower ie. when it shows I am doing 60 I am actually doing 58k, my dash cam will show 60k.