How to get legal advice in traffic matters.
Speedometer Inaccuracy, Radar tolerances and the Law


Speedometer Accuracy

If a speedometer is not accurate, it could display a lower speed than the vehicle is actually travelling at, thus causing drivers to believe they are not speeding when in fact they are. This effect can occur if a speedometer under-reads true speed.

  • A speedo over-reads if it displays 100kmh when the vehicle's actual speed is 90kmh.
  • A speedo under-reads if it displays 100kmh when the vehicle's actual speed is 110kmh.

Your actual speed can be easily determined these days using GPS (Global Positioning System). I have a GPS unit which can display speed to the first decimal place. There are numerous apps available for iPhone (or similar) that display speed using the phone's built in GPS. Motorists should have their speedo calibrated to ensure it is as accurate as is reasonably possible. Up until July 2006 the Australian Design Rules required new cars to have speedos that are accurate to within 10% of actual speed. The current Rules disallow under-reading, and permit over-reading by up to 4kmh + 10%.


Speedometer Errors

Several factors can affect the accuracy of speedometers:

Worn tyres: As the rubber on the tyre wears, the tyre diameter becomes smaller. This will make the wheel travel a shorter distance per revolution. Your speedometre might think one revolution is 170cm, but due to tyre wear it is in fact 167cm. So a car running at the same RPM will actually travel less distance. Instead of covering 100km in one hour, it goes only 99km. As the rubber wears, the vehicle's actual speed drops relative to the displayed speed. Because speedos should always be calibrated when tyres are new, not when they are worn, driving on worn tyres will mean your speedo over-reads. Actual speeds will always decrease relative to a fixed displayed speed as the tyres wear. Having worn tyres can not cause you to speed inadvertently.

Tyre Pressure: If you decrease the tyre pressure, the diameter of your wheel decreases and the wheel will travel a shorter distance per revolution. This will decrease the actual speed of your vehicle in comparison with the displayed speed. If you drive with low pressure in your tyres you reduce the risk of unknowingly driving over the speed limit. Because speedos should always be calibrated when tyres are fully inflated, driving on low pressure tyres cannot cause your speedo to under-read. However, heat build-up or over inflation could possibly cause the tyres to balloon slightly which can increase their diameter and could theoretically cause under-readings if there were no other factors offsetting this error.

Rim size: If you change the diameter of your wheels you will affect the accuracy of the speedo. Changing from 16 inch wheels to 17inch wheels does not necessarily change the total diameter of the wheel. It depends a lot on the type of tyre that is used. Changing to a wider diameter rim will usually result in a larger diameter wheel. A larger wheel causes your car to travel further with each revolution. If the speedo is not recalibrated it may under-read, which will mean your actual speed might be greater than that displayed on your speedo. If you are increasing the rim size of your tyres, you should recalibrate your speedo to make sure you do not inadvertently exceed the speed limit.

Differential ratios: Changing the gear sizes in your vehicle's differential or gear box can affect the accuracy of the speedometer, resulting in either over-reading or under-reading depending on what the change was.

Vehicle load: When you load your car the tyres carry more weight, which can cause them to depress slightly and have a smaller diameter. This can cause an over-reading. It can never cause under-reading. It can not cause you to exceed the speed limit unknowingly. Just as you calibrate your bathroom scales when there is no weight onboard, so too are car speedos calibrated when there is no extra load on the tyres.

Speedo Displays: Many modern speedos display in increments of 5 kmh and do not easily display speeds down to 1kmh accuracy. The needle mechanism has its own accuracy limitations. Some old vehicles will have faulty speedos or they have fallen out of calibration, or the needle may wag up and down slightly making it difficult to determine exactly what speed is being displayed.

Speedo Needles: The size and shape of needles can make it difficult to tell exactly what speed is displayed. Given the variables mentioned above, the observable displayed speed is a reasonably accurate, and usually conservative, estimate of actual speed.

These potential errors have been widely known for decades.


Australian Design Rules (ADR)

When vehicle manufacturers install a speedo at the factory, the speedo has been calibrated using data that assumes the vehicle is fitted with new standard tyres inflated to full pressure, on standard rims, and usually the tyre circumference has been measured before the wheel is put on a vehicle or when the vehicle is on a hoist (no load on tyre). When they test the car for compliance with the ADR, the speedo will invariably produce an over-read result.

The current Australian Design Rules require speedos to be calibrated when the vehicle is unladen, fitted with normal tyres inflated to full pressure with an allowance for tyre heating:

"Unladen vehicle" means the vehicle in running order, complete with fuel, coolant, lubricant, tools and a spare wheel (if provided as standard equipment by the vehicle manufacturer), carrying a driver weighing 75 kg, but no driver's mate, optional accessories or load.
"Tyres normally fitted" means the type or types of tyre provided by the manufacturer on the vehicle type in question; snow tyres shall not be regarded as tyres normally fitted;
"Normal running pressure" means the cold inflation pressure specified by the vehicle manufacturer increased by 0.2 bar.

The current ADR prohibits any under-reading:

"5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2).
0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h."

This formula means that the vehicle's actual (true) speed must not be greater than the displayed speed. (Displayed speed minus true speed must be greater than or equal to 0, and less than or equal to 4kmh plus 10% of true speed). This means that if your vehicle's actual speed is 100kmh, the displayed speed is permitted to be anywhere between 100kmh and 114kmh.

Prior to July 2006 the ADR allowed the speedo read 10% of true speed. This means that cars sold new prior to 1 July 2006 could comply with the ADR even if the speedo under-read by 10%. Despite this being theoretically possible, due to the testing procedures and the reasons stated below it was unlikely to occur in practice.

The reason the ADR are relevent to the debate is that some people argue that if the ADR allows cars to have speedos that under-read by up to 10%, then surely our road laws should tolerate people relying on their speedos and possibly driving up to 10% over the speed limit. Therefore, the argument goes, we should not be fined if we travel at 110kmh in a 100kmh zone because we are travelling within the tolerances allowed by the ADR. Such a situation would be quiet generous to motorists and would make it impossible for anyone to allege that they were mislead by their inaccurate speedo.

You may find some people who argue that the Australian Design Rules somehow have the equivalence of Commonwealth legislation which means they can override Victoria's speeding laws. This is utter stupidity. First, the ADR is not Commonwealth legislation, even though it may have been adopted by some Commonwealth legislation. Even where the ADR is incorporated into Commonwealth legislation, neither that legislation nor the ADR itself conflicts with any Victorian law that requires a motorist to drive within a speed limit. This means there is no conflict of laws that requires s.109 of the Australian Constituion to resolve. 

Another problem with this argument is that it ignores reality. It assumes that speedos are under-reading by 10% when the truth is the vast majority of speedos over-read by about 3%. So in effect, it is allowing the majority of motorists the opportunity to drive at 10% above the speed limit. If the speed limits are there for a reason, then they would all need to be reduced by 10% to counter the effect of motorists increasing their speed above it. 

If a motorist has a speedo that under-reads by any amount, then that person could try to argue that they have a defence of "honest and reasonable belief" in a state of facts which if true would make their conduct innocent. (See Proudman v. Dayman). Unfortunately, the Victorian Supreme Court has said that your state of mind (i.e. what you believed) is irrelevant in all speeding cases. So what you thought or believed can never be a defence to a speeding charge. Anyway, a Proudman v. Dayman defence would never be dependant on the existence of the Australian Design Rule specification. A court would be concerned with what your speedo actually displays, not what the ADR says it could display. The existence of the ADR actually harms an argument based on honest and reasonable belief.  If the ADR says your speedo can lawfully be inaccurate at the point of sale then it is not reasonable for you to believe that it is accurate when you drove the car.

Any driver who relies on the existence of the old ADR as the basis for their belief that their speedo was accurate will fail because the old ADR does not require a speedo to be accurate. It allowed it to be out by up to 10%.  And if you're thinking of arguing that the ADR is a "law" which applies in the State of Victoria, then you'll need to think of an answer to the principle that ignorance of the law is no excuse. If the ADR is a law then you are deemed to have known that your speedo could be under-reading by up to 10%, so you had no basis for your belief that it was correctly displaying your speed at the time of the alleged offence. 

If having an incorrect speedo were a defence to speeding then all you would need to do is ensure your speedo is wrong and you will never have to pay a fine. If the inaccuracy of your speedo is merely a theoretical possibility, then that just turns a very weak argument into a hopeless one because you are not addressing what in fact happened to you on the date of the offence.

For most of the 20th century there was no requirement that cars must be fitted with speedos, seat belts and many other items we all take for granted these days. 


Relevance of speedo errors in court

Having an inaccurate speedo will never help you in court, but an accurate speedo might. Some people claim the inaccuracy of speedometers could be used in court as a defence to a speeding charge. The argument is that the driver will allege that he or she was mislead by the incorrect speedo. Clearly an over-reading can not be relevant in any defence, because then the driver would be travelling slower than the displayed speed. You must have an under-reading if you want to argue that you exceeded the speed limit because you were mislead by your speedo. 

Traffic offences usually fall into two categories. There are absolute liability offences and strict liability offences. 

An absolute liabilty offence is one where the prosecution needs to produce evidence of what someone did only. What the accused believed is always irrelevant. So the only thing that matters is the actus reas (actions) while mens rea (state of mind) is always irrelevant.  In Victoria abolute liability offences include disobeying traffic signs, exceeding the speed limit and drink driving. See: He Kaw Teh.

A strict liability offenceis one where the prosecution intially needs evidence of what you did only, and does not need to adduce any evidence that you were aware of the factual circumstances of the offence. But if during the hearing of the case your lack of knowledge becomes an issue then the prosecution carries the burden of proving that you knew or ought reasonably have known about the factual circumstances of the case. For example, driving whilst suspended is a strict liability offence where it might be raised in the case that the accused did not know his licence was suspended when he drove the motor vehicle. If that happens the prosecution carries the burden of satisfying the court beyond reasonable doubt that the accused did not have such a belief.  The mistaken belief needs to with respect to an action or activity and must not be a failure to understand or know the law. Ignorance of the law is never a defence. Examples are driving whilst disqualified and dangerous driving. See: Kidd v. Reeves.

Most motor traffic offences are absolute liability offences where the driver's state of mind is never relevant. In Victoria, the Supreme Court has said in the case of Kearon v. Grant that the offence of exceeding the speed limit is an absolute liability offence.  That means, not only does the prosecution not need to prove anything about the driver's knowledge, but the accused can not even raise the question of their state of mind as a defence. It is never a defence for a driver to say "I did not realise I was speeding" or "I honestly and reasonably thought I was driving under the speed limit".

Generally the harsher the penalty is (e.g. punishable by imprisonment), the more likely the traffic offence will be a strict liabilty offence rather than an absolute liabilty offence, although in Victoria drink driving appears to be an exception to this rule. 

Despite speeding being a strict liabilty offence some people still try to rely on their faulty speedo as a defence in court. Some people claim the new ADR helps to support a defence of honest and reasonable belief, and that would be right if speeding were a strict liability offence instead of being an absolute liability offence.

Ignoring for a moment that speeding is an absolute liability offence, to win using this argument it would be necessary for the driver to show that they had an honest belief that they were driving lawfully at the time, and that it was reasonable for them to hold that belief. It is not an easy defence to run.

If you modify your vehicle by changing wheel sizes or gear ratios, then it is reasonable to expect that the speedo will become inaccurate and may under-read true speed. Any court will say that a driver who chooses to drive without checking whether the modifications have affected their speedo, or without having the speedo re-calibrated is reckless as to the accuracy of their speedo. In those circumstances the driver should not expect the court to accept that it was reasonable for the driver to form a belief that he or she was driving within the speed limit based on the display of an inaccurate speedo.

The usual wear and tear factors outlined above all work to increase the displayed speed (making it display a speed greater than true speed), and therefore can not form the basis of an honest and reasonable belief defence. There is no defence available unless the speedo has erroneously displayed a speed lower than actual speed. i.e. the speedo displays 100kmh when your actual speed was 115kmh. Tyre wear, carrying heavy loads and low tyre pressure all result in over-readings and thus defeat the argument.

In order to obtain any significant under-reading in your speedometer you will need to modify your vehicle. Fitting tyres with thicker tread or over inflating will barely account for the 1% - 2% deduction already built into speedos at factory. If you are driving a modified vehicle no magistrate is going to accept that it was reasonable to assume the speedo would remain accurate.

If you have purchased a second-hand car, it may be possible that you did not know that any modifications had been made. It is still a hard task to convince a magistrate that it was reasonable for you to trust that the speedo was accurate on your second hand car, particularly when they are not calibrated as part of the RWC testing. If you get your speedo checked and find it is under-reading, then you no longer have any basis for trusting that it is accurate.

If you want to allege that it is too difficult to read your speedo down to 1 kmh (because of the graduations on the display, or the size of the needle), then the court will expect you to err on the side of caution when driving. In any case, all that is required is for a speedo to indicate if you are over or under 60kmh or 80kmh, for example. There is no requirement, either legal or practical, for a speedo accurately to display a speed of 87kmh, because we do not have any 87kmh zones in Australia. The speedo simply needs to indicate if you are over or under the speed limit. So claiming your needle or display makes it hard to determine if your speed was 102kmh or 104kmh is not going to get you anywhere. A court will expect you to keep the middle of the needle below the 100kmh graduation mark when in 100kmh zones, and that a person who drives over the limit because they could not clearly read the exact speed must have been reckless as to whether or not they were committing an offence. This is not a defence. The court expects drivers to make sure they are driving within the law.

Defending a speeding charge based on an incorrect speedo.

The Victorian Court of Appeal decided in 1990 (Kearon v. Grant) that speeding offences are absolute liabilty offences for which a defence of honest and reasonable mistake does not exist. All Magistrates Courts are obliged to follow and apply the decision of Kearon v. Grant. This means your belief as to how fast you were driving can not offer you any assistance whatsoever in defending a speeding charge in any court in Victoria.  The court can consider evidence of how fast your vehicle was going. It can not accept evidence of how fast you thought it was going.

If you want to try to overrule Kearon v. Grant (by taking your case all the way to the Court of Appeal or the High Court) you will need to rely on the Proudman v. Dayman defence. You will need the following: a relatively new car (factory configuration, not worn out), an engineer's report that shows how much the speedo under-reads, the opinion of an expert to say that at the time when the vehicle was being driven at the speed detected by the police, the vehicle's speedo displayed a speed that was within the speed limit, and evidence from the driver to say that he had an honest and reasonable belief that at the time of driving he believed his speedo was accurate, and that it displayed a speed within the speed limit at the time of the offence and that he relied on his speedo to determine what speed he was driving at. 

A court is unlikely to accept this defence if your actual speed was so high that even an inaccurate speedo would have displayed an illegal speed, or if you did not have good reason to trust your speedo was accurate, or if you can not prove your speedo is in fact under-reading. You will need to run this case through the Magistrates Court, then the Supreme Court and then to the Court of Appeal (and possibily even to the High Court) if you want to over-rule the decision in Kearon v. Grant.

Before you embark on court proceedings in a case like this, you might wish to try writing to the Penalty Review Board at the Traffic Camera Office (Level 1, 277 William Street, Melbourne)  enclosing a copy of the engineers report and calibration test results, and ask them kindly to withdraw the notice. It has been known to happen and if it happens to you please let me know.


Police allowances

Victoria Police guidelines require a deduction of 2kmh or 3kmh from the speed reading displayed by prescribed speed measuring devices, depending which device was used. This is to avoid contested cases where the only argument in court is in respect of the ability of the police device accurately to measure speed down to the last 1kmh. The deduction they allow varies for each device depending on the calibration tolerance required by the legislation for that device. As a rule of thumb you can assume mobile speed cameras have a 3kmh or 3% deduction, fixed point-to-point cameras have no deduction while all other speed detectors have a 2kmh deduction. The legislation does not require any deduction to be made, it just requires the device to be accurate to a certain degree. The 2kmh or 3kmh deduction allowed by the police is to take into account potential inaccuracies in the police speed measuring equipment. i.e. the laser or radar device might be 1% over-reading. The deduction allowed for by the police is not intended to provide any allowance for potential inaccuracies in motorists' speedos, although police may allege that it has that effect.


An unfair system

The above is an analysis of the law as it is at the moment. Debate may rage about whether police guidelines, or the law, should give greater allowances to motorists. There is nothing wrong with having speed limits on Victoria's roads, and nothing wrong with enforcing those limits. A line needs to drawn somewhere. The real issue is with penalties. Speeding fines can be easily incurred from a moments lack of concentration on speed signs, or on the speedo. The harsh consequences of licence loss from excess demerit points should not be inflicted on people who are detected driving 4kmh over the speed limit.  A much fairer solution would be to issue small fines (up to $70) with no demerit points for speeds up to 9kmh over the speed limit. (This is similar to what they have in W.A.) 1 demerit point is fair for 10kmh to 15kmh over the limit, 3 points for anything over 15kmh above the limit. In Victoria drivers more than 25kmh over the limit get mandatory licence loss, so why do they have to suffer up to 8 demerit points as well? Demerit points should only be recorded against people who are avoiding immediate licence loss. At present in Victoria, drivers travelling 35kmh over the limit get 6 months licence loss plus 6 demerit points, which means they risk suffering suspension twice for the same offence. This is quite unfair. Drink drivers get demerit points only if they keep their licence. If they lose their licence, they get no points because they are already suffering a licence loss punishment and should not have to do it twice. Mandatory licence loss laws in Victoria leave the courts with no discretion to apply a more reasonable and fairer penalty to suit the circumstances of an offence or of the offender. Perhaps the courts should have more discretion in licence suspension and cancellation cases so that the usual sentencing principles can be applied in those cases.

 Speedo Accuracy, faulty speedo, speedo inaccuracy, inaccuracies, speedo errors, speedo margins or error, wrong speedo reading, erroneous speedo, defective speedo, mph speedo, broken speedo, speedo misreading, speedo false reading. speedometre

Related Pages:

Getting Legal Advice
Demerit Points

defective speedo, speedometer error, margin of error, uncalibrated speedo, non-calibrated, unreliable speedo, misleading, without a speedo, tolerence, tolerance, radar tolerance, lasar tolerance, radar tolerence, laser tolerence, calibration of speedometer, broken speedo,





Home . About me . Contact . Disclaimer . Site Map

Copyright S. P. Hardy