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Automotive GPS Speed Accuracy vs Speedo


The police do not prove anything. It is on you to prove it to their satisfaction at your own cost. Since most speeding tickets are from cameras you don’t get to negotiate speed with the police. Is anyone authoritatively aware whether a speedometer certification showing a real error with OEM tyres and wheels is sufficient evidence to have a speeding ticket dismissed, first time or any time?

In general if you have not had any infringement in 2-3 years, state dependent, you can plead guilty, request forbearance and promise to try harder, and the fine/points can be waived. If you contest it you are up for all costs plus the fine and points.


The rules state for any vehicle manufactured after 2005 the speedo speed indication MUST NOT be less than the true speed and it may indicate a higher speed that is worked out by a formula (the following from the Australian Design Rules) " 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 means if your speedo indicates 60 kph you may actually be travelling at a speed less than 60 kph but NOT over and it isn’t a straight 10% error allowed).


When they first made it available for civilian purposes, I seem to recall they blurred accuracy to within 100 metres. At least they have improved a little :wink: .


“Selective availability” … there are some interesting stories relating to world events around the time of the deployment transition from Block II and Block IIA, selective availability, the reason(s) it was discontinued and fun terms like “satellite co-visibility” … Timing is everything, and knowing when you’ll know where you are and where they are … fun stuff.

More recently the (eventual) coming of age of GLONASS and receivers that could meld both sources has given a much higher perceived accuracy …

Of course, YMMV (your mileage may vary) … literally …


The whole issue with inaccurate speedometer is that fuel usage per km must also be incorrect no way of ever achieving stated km/l even though you are going slower than indicated on speedo. Further more if our police only give 3% (Vic) tolerance automobile manufacturers should be held to the same requirement a $50,000 car and the speedo is out.All European trucks have the ability to set the correct speedo reading using tyre diameter ang final drive ratio down to 1km/h so cars have no excuse to not be able to adjust speed reading as they have various tyre sizes across models


That 3% allowance is when your vehicle is over the speed limit, if your car, bus, bike or truck is indicating 60 kph in a 60 kph zone then you will not be over in your true speed. You will be at 60 kph or less and so shouldn’t need to worry about the police allowance. If however you are travelling with larger than standard tyre sizes (not wider but taller) then your speed indicated on your display/dash would be lower than your actual speed and you would need to worry.

Yes they could have more accurate readings but that would also mean people would have to adjust the speedo readings regularly to account for tyre wear (smaller tyres). The way the ADR (Australian design rules) work in this regard is that from brand new to replacement of the standard tyre for the vehicle the speed of the vehicle will never be exceeding any given speed limit if the speedo is indicating that speed. So if drivers/riders want to avoid the fine they need to keep to the speed limit as determined by their speedometer and they will not be exceeding the real speed as set by law. A lot of drivers however do exceed the speed indicated and they then suggest the fine they receive for exceeding the limit is “revenue raising” when in reality they just choose to go faster than they should and so suffer the consequences of exceeding the limit.


Some do, but not so many years ago Victorians were getting fines in the mail from very minor infractions caught at road speeds. Is going 103 on a short down slope in a 100 zone dangerous, or just technically dangerous because the bureaucracy decided so?

Victoria was adamant such drivers were scofflaws and deserved the full weight of the system applied to them. That approach has since died down so I surmise saner heads prevailed, excepting that eg Victoria changed the camera/radar rules so they can be at the bottom of a hill hidden behind the bushes. One could reasonably question the statement that people always consciously chose to break speed laws.

Most (not) everyone tries to go on the posted limits, not 5 km under them, as registered on their speedo. The normal terrain in places causes speed to vary as one goes up hill and then down. Supporting the brake renewal industry is good and creates work, but. That also supports state treasuries as they include fines in their budgets, and meeting budgets is important you know. Or could it just be rule making by people eg in Melbourne who have never been in local terrain and only have been out and about in the Nullarbor where you can set the cruise control, tie the steering wheel, and pretend you are in a self driving smart car for many kms.


In my view, most of the time it’s simply ‘illegal’, ‘dangerous’ is the label that sells the concept. I strongly doubt that proper testing and evaluation is done everywhere - the posted limit is a compromise and lowest common denominator, with yellow advisory signs applied as band-aids. Terminology is carefully chosen to support that - for example “speed was a factor” is often mentioned, even “excessive speed was a factor”, both ambiguous as few accidents happen where both vehicles are stationary and similarly excessive speed is not necessarily above a given posted limit, in a given context (heavy rain, fog, etc) - less common are terms like “exceeding the posted speed limit was a factor”. I don’t think the average punter distinguishes much between these terms when reading the news, they just move on to the cartoon strips and the crosswords …

Where I am we had a return to de-restricted speeds on a few hundred kilometres of road for a couple of years. Accidents happened, but no more than usual and they involved the usual suspects. To the best of my knowledge none of them were the clear result of reckless speed, and there certainly weren’t any deaths attributed to same. None the less, with the return of the socialist government the limits were reimposed in the name of ‘road safety’ … of course everyone knew it was tit-for-tat between the socialists and capitalists. Anyone who has been here for any period of time knows where the real problems lie :slight_smile: we see them daily.

My views only …


If you drive at the very limit of the allowed speed and breach that then you have indeed broken the law and a penalty applies if caught. The posted speed limit is a maximum for when conditions of the road and traffic are prime. How many times are they prime?

The upper limit imposed means that a person should travel at a speed slightly lower and with any speed creep should still be safe from getting fined, but most push the envelope and certainly from my observations exceed the limits many times and not just by a few kilometres per hour. If they hide the camera maybe that does catch more people but if the public traveled under the speed limit they would not be fined for exceeding the limit. Also taking into account that the speedo would be either indicating the true or a higher speed than the true one, then to be fined means by their speedo they must have exceeded the speed limit. For that I have no sympathy! Governments know people will push the boundaries and so can almost lock the exact amount the fines will earn into their budgets (I guess there would be a small margin of error but they would have honed their accuracy over the years). So the easy answer for a driver/rider wanting to avoid fines is to travel under the indicated speed limit, but I think many will argue why should they as they are great drivers/riders and it is just all the other people on the road who don’t know how to drive.

@draughtrider NT townships aside when it is dry the roads outside the towns tend to be in very good repair and quite wide including the roadside allowances with little traffic compared to a busy urban street or more built up population area. In those circumstances the unrestricted speeds work well.


@grahroll, I never would have realised if you do not speed you would not get fined. (/sarcasm)

The reality is that the vast majority of drivers go at the posted limit, not under, and that is reality. Driving below the posted limit causes others to push the boundaries to get around them, and encourages road rage.

We will likely agree to disagree on the merits of hard edged ‘road safety’ that is unsupported by any study of the appropriate speeds on any particular road.

I guess I will just dutifully watch my speedo in the future, and pay no attention to the road or anything on or near the road to assure I do not transgress.

disclaimer: I have not had a moving violation since the late 1980’s when I got caught by a radar trap bordering the end of a school zone in the USA, literally 1 minute before the school zone expired whilst starting to accelerate from the reduced speed. Yes, guilty as hell, but.


Yes the majority push the limit and then they get upset when fined. Whose fault is that? I am not sure if it has been a culture of police and other road authorities turning a blind eye to minor infractions in the past so it has created a culture of poor road behaviour, or perhaps poor driver training or lack of retraining and testing, or that some do not know how to drive in a legal manner or some other reason.

I took that you knew that not speeding would avoid fines but I had put in my response that the reason for why people are fined is because the culture is to push up to if not past the boundaries whether it is speed, parking, indication, distance between vehicles while traveling and so on. The excuse about keeping an eye on the speedo is always a herring (red) about driving. If the driver cannot ensure they are not going to exceed the speed then they need to slow down a bit more to ensure they keep within limits or stop driving, I also note that the police do make allowances albeit small ones for breaking limits as we all make errors in our judgements. Hiding the camera is not the problem it is the drivers who exceed the limit who are the root cause of the problem and the camera is a result of that. Road rage is another issue as some people do not tolerate others impeding them in any way, again a sign of some breakdown in social conscience.

As to the road safety aspect, I agree that some speeds we are obliged to obey seem silly/stupid in the situation/place/time but they are in place and we as drivers/road users are given a licence to use the roads while complying with the law and if we break it no matter how close it is to a time limit, speed range change or other circumstance then we can be penalised…a line has to be drawn somewhere or do we keep moving the goal posts for each individual driver?


Lucky it wasn’t Texas ! :wink:

Nah, Survival of the fittest :wink:

ps - Cundalini wants his hand back …


But I am no longer like Mad Max lol


I disagree with that as I have found myself watching the speedo far more than I should need to because the system has made enough of us paranoid about minor infractions that we focus on avoiding infractions, not core driving.

Rules and enforcement that do anything to distract one from attention to the road are questionable but I allow that a line needs to be drawn somehow. But it is interesting you think it a red herring when most accidents result from inattention to the road and other road users, not speed in isolation; speed only kills and damages only happen when you hit another object or totally lose control and roll.

I invite you to come to my part of NE Melbourne and drive around a bit to have a reality check. It would suggest the placement of cameras at the bottoms of inclines and behind bushes can be nothing other than a revenue trap without sufficient leeway (the leeway is secret). I can be driving along at 65 in an 80 zone on a nearby main road, and going down one hill foot off the accelerator my car will go to 85 just before the bottom if I do not apply brakes, if that makes any point, just in time to apply the accelerator to go back up the other side. Hide a camera at the bottom for road safety? or revenue?

Methinks we have taken this well off topic of speedo accuracy and agree to disagree, so I’ll drop out here. We appear to have quite different views on this extended topic. I suspect a dedicated thread would become somewhat religious comparing the letter of law against pragmatism and reality so I expect I would give such a thread a miss.


You knew! The Woodlands, TX, 77381 to be precise. The final funny was that I had the option to attend an approved safe driving class in lieu of the fine and demerits since I had no priors. I found an approved class presented by a local comedy club. It was OK. I passed :slight_smile:


When speedo of our new car was seriously out we complained and it was pointed out to us that under ADR that it was okay because it was over-reporting the speed. In a 110 kph zone with the speedo showing 110 kph we were doing a bit less than 100 kph (bless those spots with the five filometre ‘check your speedo’ markings).


It gets worse as the speed increases - 10% error at 300 is 30 km/h (unsurprisingly) … there are devices that can address this, like the ‘speedo healer’ for bikes which works really well. Note: I’d recommend this kind of activity for the track or de-restricted speed zones, which no longer exist in this country …


Just back from driving several thousand km in USA. Clearly USA legal requirements are different to here.
The rental had about 20,000 miles on the clock, so tyres not so new. It didn’t have nav system. The speedo varied by no more than 1mph to the phone GPS speed. Mostly the same. Additionally, it wasn’t unusual to go by a couple of dozen radar speed signs (“Your speed is …”) per day. All speeds aligned.


Comments above regarding GPS being inaccurate accuracy for speed are not correct.

GPS absolute accuracy has a number of factors limiting the accuracy of the vehicles real position, but GPS has a high relative accuracy, so the actual GPS calculated distance travelled by the car between 2 points is very accurate. GPS speed beats your cars speedo hands down for accuracy


It would be interesting to know from what your comments are based, as it goes against published information in relation to accuracy of GPS…see the links in this previous post…

With the accuracy presented in this information, it can be easily seen that a GPS is a less relaible speed indicator than a car’s speedometer.

One of the challenges is that car manufacturers can’t sell cars with speedometers which show speeds less than actual speeds (e.g. speedometer shows 100km/h when actaul speed is 105km/h). To overcome any potential issues, the speedometer is a car conservatively shows speed and in most cases, the speedometer speed is more than actual speed (could be out by a few or more percentages).

Many in the community are aware of this and think that they can use a GPS to claw back this over reporting of speedometer speeds…but doing so is fraught with danger as outlined in the earlier post and why the courts don’t allow GPS speed data as admissable evidence.

One way to check ones speedometer is to use the surveyed in distance signage (speed distance check signage) and then calculate the error for ones own vehicle based on the difference between the speedometer reading the the signage (usally set over 5 km). It is also worth noting that the calculated error is only valid for the period around the time of doing the speed check as things like tyre wear (or changing tyres) can affect speedometer readings. This is why t is good to check ones speedometer accuracy using the signage a few times a year or when say new tyres have been added to the car.

If one purely uses a GPS for managing vehicle speed, it is likely that at some time one may receive an infringement notice/fine + points penalty as the GPS is not accurate enought to rely on. There have also been challenges to speeding fines where GPS data has been tabled and ultimately dismissed by the courts. This also says something about the reliability of GPS readings. This recent paper also provides a view on GPS vulnerabilities.

If the accuracy of GPS were submeter at all times (such as using a secondary signal to improve accuracy), then the situation in relation to speed accuracy may be different. Car based GPS don’t offer such facilities.