Is your speedo wrong? Good chance, warns expert

Riley Riley

Ever noticed how your car speedo and satellite navigation report different speeds?

If the answer is yes, then which one can you believe?

Road safety expert, Emeritus Professor Michael Regan, explains.

In Australia, he said motor vehicles must comply with what is known as Australian Design Rules (ADR).

These specify how a new vehicle should be designed and made fit for purpose if it is to be sold here.

You might be interested to learn that the ADRs prohibit manufacturers from under-reporting the speed of a vehicle.

As a result manufacturers often calibrate speedometers so that they reads above the actual speed of the car, so that they show the car going faster than it actually is.

Professor said most manufacturers do this to avoid any chance a car might be travelling at a speed higher than the reading on the dashboard.

“ADRs 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,” he said.

“This means the speedo is likely to read 100km/h when, in actual fact, your real speed is 95km/h.”

The speed displayed is determined by a vehicle’s tyres.

Speedometers are calibrated based on the rate of revolution of the car’s power train.

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

But over time Prof Regan said tyres are subject to normal wear and tear and become smaller in circumference.

This changes the accuracy of the speedometer which again shows a higher figure than the actual speed.

“If you’ve had tyres on your car for a long time and the tread is worn, the wheels revolve faster than they would be if your car was fitted with brand new tyres,” he said.

“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.” 

Satellite navigation on the other hand uses global positioning satellite (GPS) technology to calculate speed.

It determines the time taken to travel a given distance and as a result, GPS speed is often hailed as being more accurate.

Prof Regan said there may be a very short time lag as the GPS re-adjusts, but it’s so insignificant that drivers probably won’t notice it.

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

“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. 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 relatively rare 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.

Prof Regan said to be absolutely sure how fast you’re driving, you need to know how much the car’s speedo is out by.

Vehicle manufacturers could use the GPS system that is inside virtually all modern cars to calibrate the speedometer more accurately, but 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.”

Speed signs used for road projects or in school zones display your speed as you approach.

They use radar to measure the time taken between sending and receiving signals from a car at one point.

This time difference is converted into distance, the process is repeated again and the radar speed signs calculate the new distance.

Your speed is calculated based on the two different distances and this is then displayed on the sign.

Prof. Regan said some drivers may find that the detected speed can be different from the one showing on their 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 speed and increasing the number vehicles adhering to the speed limit. 

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

Prof. Regan said the next important step in car safety technology is implementing intelligent speed assistance (ISA) systems.

They 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 to detect and read traffic signs and lets the driver know in real-time what the speed limit is.

They come in two basic forms.

Advisory systems issue a warning if the driver exceeds the speed limit and have been in existence for more than two decades.

“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.

“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, however, the European Road Safety Charter has made it mandatory for all new vehicles entering 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 permanently,”he said.

Professor Michael Regan
Professor Michael Regan


CHECKOUT: Event recreates horror of fatal crash

CHECKOUT: Safety first, or was that Cornell first?


Your email address will not be published. Required fields are marked *