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There are valid reasons why vehicle speedos aren't as accurate as GPS.


A University of NSW road safety expert revealed the truth about why speedo readings can be different from GPS measurements.


Have you ever noticed how sometimes the display on your vehicle’s speedometer is different from the speed shown on the navigation app on your phone? 

You’re not alone. And it’s all to do with ADRs. 

The Australian Design Rules (ADRs) – set by the Department of Infrastructure, Transport, Regional Development, Communications, and the Arts – are the national standards for road safety and specify how a vehicle should be designed and made fit for purpose when it’s sold in Australia. 

According to the ADRs, manufacturers are prohibited from under-reporting a vehicle’s speed. As a result, vehicle manufacturers often factory-calibrate speedometers so that they read above the actual real vehicle speed. 

Road safety expert, Emeritus Professor Michael Regan, said most manufacturers do this to avoid any chance that the vehicle might be travelling at a speed that is higher than the reading on the dashboard. 

“ADRs require a speedo tolerance of zero percent under, to 10 percent above, the actual speed, so manufacturers typically set it at about five percent over,” Prof Regan said. 

“This means a speedo is likely to read 100 kilometres per hour when, in actual fact, the real speed is 95 kilometres per hour.” 



What determines the speedometer reading 


“The displayed speed is determined by the vehicle’s tyres,” said Prof Regan. 

“Speedometers are calibrated based on the rate of revolution of the vehicle’s powertrain. 

“This, in turn, depends on the tyres and it’s usually on a set of new tyres of a certain circumference,” he said. 

“When the manufacturer carries out speed calibration tests, they’re based on brand-new tyres. 

“But over time, as the tyres experience normal wear and tear, they get smaller in circumference. 

“This changes the accuracy of the reading of the speedometer – again showing a higher speed than the actual speed. 

“So if you’ve had tyres on your vehicle for a long time and the tread on the tyre wears away over time, that means that the wheels are revolving faster than they would be if your car was fitted with brand-new tyres. 

“So as your tyres get older, your speedo actually overestimates your speed, so you might think you’re going faster than you actually are. 

“In any case, if the tyres are worn enough to make a noticeable change to the speedo reading then it is likely time to replace them.” 



What about the speed on my GPS? 


Many drivers use mobile navigation apps that also measure and display road speed. 

Unlike the speedometer, these apps take advantage of global positioning satellite (GPS) technology to calculate speed by determining the time taken to travel a given distance. 

“As a result, GPS speed is often hailed as being more accurate than the car’s speedo,” said Prof Regan. 

“While there may be a very short time lag as the GPS calculations re-adjust, it’s so insignificant that drivers probably won’t notice it,” he says. 

“If you’re driving on a flat, straight road, the GPS is likely to be more accurate than what’s displayed on your speedo. 

“However, if you’re going up or down a steep hill, the actual speed – for example, as measured by police mobile radar – will usually be greater than the GPS value, but proportional to the steepness of the road you’re travelling on. 

“It is the change in elevation, relative to the GPS satellites circling above, that results in the error, so horizontal bends do not affect it. 

“In theory a clever GPS device could account for the road steepness and adjust the displayed speed so it is more accurate. 

“However, this is a relatively rare situation and there is no strong justification for navigation devices to make this adjustment, but drivers should bear this factor in mind when driving on steep roads. 

“To be absolutely sure how fast you’re driving, you need to know how much the car’s speedo is out by.” 

So why don’t vehicle manufacturers use the GPS navigation system that is inside virtually all modern cars to calibrate the speedometer more accurately? 

Prof Regan said current regulations do not require or encourage it. 

“In the future, I hope this changes, because drivers would want the most accurate reading to know how fast they’re actually going. 

“This feature could also automatically adjust for tyre wear and replacement tyres.” 



Radar speed feedback signs 


Radar speed signs are used for traffic management of road projects or in school zones and display your speed as you approach and drive past. 

If you’re driving at or below the speed limit, you’ll often be rewarded with a smiley face or your speed displayed on the screen. However, if you’re driving over the speed limit, a sad face or sign telling you to slow down usually appears. 

Radar speed systems measure the time taken between the sending and receiving of a radar signal from a vehicle and this time difference is converted into distance. 

The process is repeated again and the radar system calculates the new distance. The speed is calculated based on the two different distances and this is then displayed on the sign. 

Prof Regan said that the detected speed can be different from the one showing on the speedometer. 

“It’s just the way they’re set – just like how our car’s speedo is usually higher than the GPS speed,” he said. 

“These radar speed signs serve as a reminder for us to assess our speed as we approach areas with changing conditions – especially near zones where there may be more construction workers using the road. 

“In some studies, radar speed signs have been shown to be highly effective in reducing speeds and increasing the number vehicles adhering to the speed limit in the areas installed. 

“The public nature of having your vehicle speed displayed for everyone to see makes you more accountable.” 



Intelligent Speed Adaptation 


Prof Regan said the important next step in car safety technology is implementing intelligent speed assistance (ISA) systems, which have been shown in numerous studies to be highly effective in reducing speeding and speed-related crashes. 

ISA relies on GPS and/or built-in cameras on the car to detect and read traffic signs and lets the driver know in real-time what the speed limit is. 

ISA systems come in two basic forms: advisory ISA systems can issue a warning to drivers if they exceed speed limits. Such systems have been in existence for more than 20 years. 

“More advanced, limiting-ISA systems can physically prevent the vehicle from exceeding the posted speed limit; like a conventional speed limiter, but a more intelligent one,” he said. 

“But, like adaptive cruise control, the driver is always in control and can easily override the ISA system. 

“This is just another example of how systems can be implemented to improve road safety, because the reality is that sometimes drivers can become distracted and miss changes in speed signs, or simply not realise that their speed has crept up.” 

Through its star safety rating system, the Australasian New Car Assessment Program (ANCAP) has encouraged fitment of these speed assistance systems for more than a decade and assesses vehicles based on the presence of ISA and its performance. But ANCAP is a voluntary program and there is no equivalent requirement in the mandatory ADRs. 

From July 2022, the European Road Safety Charter made it mandatory for all new vehicles in the European market to be fitted with advisory ISA. 

Prof Regan said: “Europe is leading the way in this area by implementing this new rule. 

“If Australia wants to get more serious about road safety, we need to bring this system to the market.” 



Speeding is never safe 


Each year, speeding contributes to about 41 percent of road fatalities and 24 percent of serious injuries in New South Wales alone. 

Prof Regan said that although speedometers are calibrated to overstate our speed, it’s not a green light for drivers to engage in excessive speeding. 

“All drivers must obey the road signs to ensure the safety of all drivers and pedestrians who use the road,” he said. 

“I think most people don’t realise that driving even a couple of kilometres over the speed limit greatly increases the risk of a serious crash, which can have devastating consequences. 

“For example, half of all serious crashes involving a vehicle travelling at only five kilometres per hour over the speed limit would have been avoided, or would not have resulted in injuries or fatalities, if that vehicle had been travelling at the speed limit.” 


























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