Finding the Truth Behind Your Vehicle’s Speedometer: Debunking the Accuracy Myth

Many drivers have long suspected that their vehicle’s speedometer does not provide an accurate reading, often displaying a higher speed than GPS navigation systems or speedometer apps. But why is this the case? The answer lies in Australian Design Rules (ADRs), which enforce strict guidelines for car manufacturers regarding speedometer accuracy. According to the ADRs, speedometers must report a speed between 0% under and 10% above the actual speed. To adhere to these regulations, car manufacturers typically set the speedometer about 5% above the real speed, resulting in readings that are higher than the actual velocity.

Emeritus Professor Michael Regan from the University of New South Wales explains that this intentional discrepancy in speedometer readings is due to the tolerance requirement imposed by the ADRs. However, there is another factor at play that contributes to the speedometer’s inaccuracy over time – tire wear. A vehicle’s speedometer is calibrated based on brand-new tires with their maximum circumference. As tires wear down from normal usage, their circumference decreases. Consequently, the speedometer continues to display a higher speed than the actual velocity.

To mitigate this issue, drivers can opt for higher-profile tires when it is time to replace their old set. Increasing the sidewall height by a few millimeters results in a larger overall circumference, effectively reducing the speedometer’s tolerance. However, it is crucial to note that not all tires, vehicles, and speedometers are the same. Before making any changes, drivers should thoroughly research and ensure that their modifications comply with legal standards. The replacement tires should not deviate more than 15mm in size from the largest and smallest sizes listed on the placard. Additionally, other specifications, such as load rating and speed rating, listed on the placard, must also be taken into account.

Alternatively, drivers can rely on GPS devices for more accurate speed readings or adjust their driving based on the known discrepancy in their car’s speedometer. While GPS systems tend to offer greater precision than mechanical speedometers, it is important to be aware of any potential limitations or inaccuracies that may apply in specific situations.

Frequently Asked Questions (FAQ)

Q: Why does the speedometer show a higher speed than GPS or speedometer apps?

A: The speedometer is intentionally set to display a slightly higher speed to comply with Australian Design Rules, which mandate a tolerance range of 0% under to 10% above the actual speed.

Q: Does tire wear affect the accuracy of the speedometer?

A: Yes, as tires wear down, their circumference decreases, leading to a higher speed reading on the speedometer compared to the actual velocity.

Q: Can changing to higher-profile tires improve speedometer accuracy?

A: Yes, increasing the sidewall height of the tires results in a larger overall circumference, effectively reducing the discrepancy between the speedometer reading and the actual speed.

Q: Are all tires, vehicles, and speedometers the same?

A: No, it is crucial to research and ensure that any modifications or replacements comply with legal standards and are compatible with the specific vehicle and speedometer.

Q: Are GPS devices more accurate than mechanical speedometers?

A: GPS devices generally provide greater accuracy, but it is important to be aware of any potential limitations or inaccuracies that may arise in certain situations.