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

You鈥檙e not alone. And it鈥檚 all to do with ADRs.

The - set by the Department of Infrastructure, Transport, Regional Development, Communications, and the Arts - are the national standards for road safety and specify how a car should be designed and made fit for purpose when it鈥檚 sold in Australia.

According to the , car manufacturers are prohibited from under-reporting a vehicle鈥檚 speed. As a result, vehicle manufacturers often calibrate the speedometers at the factory so that it reads above the actual real speed of the car.

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

鈥淎DRs require a speedo tolerance of zero per cent under to 10 per cent above the actual speed, so manufacturers typically set it at about five per cent over,鈥 Prof. Regan says.

鈥淭his means the speedo is likely to read 100 kilometres per hour when, in actual fact, your real speed is 95 kilometres per hour.鈥

What determines your speedometer reading

The displayed speed that you鈥檙e travelling on the road is determined by the vehicle鈥檚 tyres, says Prof. Regan.

鈥淪peedometers are calibrated to read based on the rate of revolution of the car鈥檚 power train. This, in turn, depends on the tyres and it鈥檚 usually on a set of new tyres of a certain circumference,鈥 he says.

鈥淲hen the manufacturer carries out speed calibration tests, they're based on brand new tyres.

鈥淏ut 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 鈥 again showing a higher speed than the actual speed.

鈥淪o if you've had tyres on your car 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.

鈥淪o as your tyres get older, your speedo actually overestimates your speed so you might think you鈥檙e 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.鈥

car tyres

Car tyres looking a little worn? It could be time to change them as they may be affecting your speedo reading. Photo: Getty Images

What about my speed on my GPS?

Many drivers use mobile navigation apps which also measure and display the speed being travelled within the interface.

But 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, the GPS speed is often hailed as being more accurate than the car鈥檚 speedo, says Prof. Regan.

鈥淲hile there may be a very short time lag as the GPS calculations re-adjust, it鈥檚 so insignificant that drivers probably won鈥檛 notice it,鈥 he says.

鈥淚f you鈥檙e driving on a flat, straight road, the GPS is likely to be more accurate than what鈥檚 displayed on your speedo.

鈥淗owever, if you鈥檙e 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鈥檙e travelling on.

鈥淚t is the change in elevation, relative to the GPS satellites circling above, that results in the error. Horizontal bends do not affect it.

鈥泪苍 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. Drivers should just bear this factor in mind when driving on steep roads.

鈥淭o be absolutely sure how fast you鈥檙e driving, you need to know how much the car鈥檚 speedo is out by.鈥

So why don鈥檛 vehicle manufacturers use the GPS navigation system that is inside virtually all modern cars to calibrate the speedometer more accurately?

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

鈥泪苍 the future, I hope this changes, because drivers鈥 would want the most accurate reading to know how fast they鈥檙e actually going. This feature could also automatically adjust for tyre wear and replacement tyres.鈥

man using phone in his car

Mobile phone apps, such as Google Maps or Waze, use GPS technology to calculate the speed of the vehicle. Photo: Getty Images

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

Radar speed feedback signs use radar systems to measure the time taken between the sending and receiving of the radar signals from a car at one point and this time difference is converted into distance.

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

Prof. Regan says some drivers may find that the detected speed can be different from the one showing on their speedometer.

鈥淚t鈥檚 just the way they鈥檙e set - just like how our car鈥檚 speedo is usually higher than the GPS speed,鈥 he says.

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

鈥泪苍 , 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.聽

鈥淭he public nature of having your speed displayed for everyone to see makes you more accountable.鈥

Intelligent Speed Adaptation

Prof. Regan says the important next step in car safety technology is implementing intelligent speed assistance (ISA) systems, which have been shown in 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聽can issue a warning to the driver if they exceed the speed limit and have been in existence for more than two decades.

鈥淢ore 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,鈥 he says.

鈥淟ike adaptive cruise control, the driver is always in control and can easily override the ISA system.

鈥淭his 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 creeped up.鈥

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 .

But ANCAP is a voluntary program and there is no equivalent requirement in the mandatory ADRs.

From July 2022, the made it mandatory for all new models of vehicles entering the European market to be fitted with advisory ISA.

Prof. Regan says: 鈥淓urope is leading the way in this area by implementing this new rule.

鈥淚f Australia wants to get more serious about road safety, we need to bring this system to the market permanently.鈥

Read more:聽

Speeding is never safe

Each year, speeding contributes to about 41 per cent of road fatalities and 24 per cent of serious injuries in alone.

Prof. Regan says that just because our speedometers are calibrated to overstate our speed, this does not give the green light for drivers to engage in excessive speeding.

鈥淎ll drivers must obey the road signs to ensure the safety of all drivers and pedestrians who use the road,鈥 he says.

鈥淚 think most people don鈥檛 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.

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