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Automotive GPS Speed Accuracy vs Speedo


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.


They are a great check tool assuming at the same time you are the only car or truck on the road for some distance and can keep a fixed 100kph or what ever speed you choose to sit on for the check. 180 seconds for 5km at 100kph you still need a stop watch function and passenger to have a reliable enough result to be within a 1kph range or approx +/- 1%.

Also note that the effective rolling radius of a tyre changes with inflation pressure and car load. It’s easy to get slightly different results. The more variables the more the little variances add up to one big error.

Good luck finding a set of mileage (kilometer distance traveled) markers to check your speed against around here, SE Qld. I used to drive in other parts of QLD and there were several set out in convenient locations.


Once again we have people telling us that in theory speedos are more accurate than GPS and the reverse. Both measures are bedeviled with many sources of error and without any solid numbers it is a guess which is better or worse.

Perhaps some measurement of real world examples would shed light on the question. If the measurements were taken over a sufficient range of circumstances by a sufficiently accurate method then the matter might even be settled. Until then it is arguing how many angels can dance on a pin head.


Fact - neither the GPS in a mobile phone nor the everyday car speedo are consistently accurate?

For a different view on speed and distance traveled you might need to consider a Halda rally trip meter used by amatuer car rally participants and or for professional WRC options such as;

Note all professional options require some form of calibration for speed?

Note no need to check the shoe size of angels or measure pin heads with any of these options.


Wow, what passions!
Here’s my take on theory vs reality.
The Quora reference above claims GPS position error “up to 5 metres”.
In extreme situations with poor satellite signal (such as inner-city surrounded by high-rise buildings) you get total signal drop-out and no meaningful result.
On open highways, particularly with modern GPS’s with both GLOSNASS and GPS receivers enabled, you can expect somewhat better accuracy.
A simple check is whether the GPS can accurately display which side of a dual-carriage highway you are on.
Aust Design Rules specify speedo’s should be between -10% thru +0% accurate. My experience with at least 20 or 40 hire cars (Toyota Corolla, VW Golf, Audi A1, A3, A4) is that all new cars on new tyres read -5% on their speedo’s (matched against two Tomtom GO 520’s, a Garmin DriveAssist 51, and two ODBC displays). That is static speed displayed in cruise control on a straight, flat road at highway speeds.
You often get a discrepancy of 1 kph and rarely 2 kph during acceleration or braking as each device seems to have a different sampling rate and averaging interval.
Different models and manufacturers of tyres and tyre wear do affect speedo and ODBC readings, not GPS.
Roadside radar speed displays usually match my GPS, though sometimes they seem affected by other vehicles or perhaps braking.


This article (May 2018) also provides information on the accuracy of GPS in Australia. …currently about 5m but the government is investing $260M (last years budget) on Satellite Based Augmentation System (SBAS), which aims to correct GPS accuracy to around a metre.

If an accuracy of a metre or less is achieved, then GPS based speed measurements will become substantially more reliable and accurate. This would bring accuracy from about 8-10% down to about 2%. A 2% accuracy is likely to be equal to or better than a cars speedometer.

Until SBAS (and using devices which can use this service) is rolled out, one is playing Russian roulette if one only uses GPS to measure vehicle speeds. Presently there is a risk that one may pass through a speed check/radar with the GPS under reading actual speed by maximum of 8-10km/hr (viz. In worst case scenario a GPS indicating 100km/h whereby actual speed is 108-110km/h).

With SBAS, worst case scenario this would come down to about 2km/h or 102 km/hr actual speed when the GPS indicates 100km/h.


GPS speed is an interesting debate, but to me it has become esoteric and technical while ignoring ‘why’ are we worried about it. When all is written, GPS computed speed is imperfect yet accurate enough for a purpose.

A speed display is to avoid an infringement. How many drivers on any road conclude ‘I should not be going over 60 kph’ here because it would be unsafe as compared to ‘I should not be going this fast here and need to slow down.’ Setting and enforcing what is usually an arbitrary limit is seen by some to be ‘part of the toolkit’ to reduce the road toll; it is certainly a revenue raiser; but if one steps back to look at what constitutes safe driving I propose GPS for speed measurement has as much merit as a mechanical speedo, or its electronically simulated counterpart, excepting it is more problematic to accept it for the purpose of issuing infringements, although that could be addressed by rational drafting of related legislation.

What is the purpose of a limit and which speed is more accurate? In the context of road safety why would 1 or even 5 kph matter? Is it because speed kills and that is an absolute above all other concerns? I suspect there will be at least one rebuttal that a crash at say X+5 kph will be more serious than one at only X kph, and nobody will change that or my alternative mindset. But avoiding a crash is about more than speed, it is about attention, reaction times, vehicular braking, keeping safe distances, and safe driving, not just ‘speed kills’. And therein lies one problem, that it is easier and less costly as well as profitable for government to get revenue from arbitrary rules than it is to patrol and take bad drivers off the roads and keep them off.

FWIW I always drive with my GPS running, and day in day out it shows a constant relationship to the digital speedometer display (within seconds of lags) depending on terrain and changes of speed. I trust it is probably a tad more accurate than the speedo at constant speed since when driving, if I use the GPS speed I am literally one of many cars travelling down the road together, but if I use the speedo I will soon be leading a parade, giving me cause to think my speedo might read a bit higher for any given speed than most. It could be everyone else on the road is technically speeding, but would it always be everyone but me?

split this topic #48

3 posts were split to a new topic: Honda GPS issues

Honda GPS issues

We had a car (purchased new, fitted with manufacturer recomended tyres) where if you wanted to do 100 km/hr in reality you needed to get the speedo to point at 115 km/hr (we measured more than once against the five kilometre marked stretches of road).
It was so bad that I made up a little translation chart and stuck it to the instrument cluster:
actual 40 = speedo 48
actual 50 = speedo 59
actual 60 = speedo 70
actual 70 = speedo 81
actual 80 = speedo 92
actual 90 = speedo 103
actual 100 = speedo 114
actual 110 = speedo 125
We complained to the dealer and were eventually told that there was a recall of faulty speedos. So our car went in and was fixed under warranty. After that if you wanted to do 100 km/hr in reality you only needed to get the speedo to point at 108 km/hr (we measured as before, and also with GPS on level straight road). So we complained to the dealer and were told that a maximum 10% difference between speedo reading and actual speed was permitted by law (ADR) as long as the speedo was overstating the speed and not understating the speed. The percentage difference after the “fix” continued to vary with speed, but as it only reached 10% and was always “in the right direction” the dealer and manufacturer told us that they had done as much as they had to (this was back in the days before ACL).
So I modified the little translation chart stuck it to the instrument cluster (and we bought a GPS for that car).
But it was a pain because we had a sister car to that one (same colour, same brand & model, different years) which had correct speedo reading, and you always had to think which car am I in - is it the one with the inaccurate speedo or not?


Totally unbelievable.Humans have lost the ability to read maps and signs and are driving into lakes oceans and off cliffs thanks to GPS. Never bothered with GPS. A colleague drove me to a worksite once using his GPS. Took 90 minutes of zig zagging. I suggested a more direct return and it took 75 minutes hassle free!


Suggest you set that GPS to shortest distance and you might visit streets and estates you did not know existed, let alone if they could possibly be on the most direct route. I do that now and again and it can be entertaining. Once I went to a regular nearby business area where there are two main roads each taking 7 minutes. The GPS cut right down the middle, saving maybe 7metres taking 12 minutes because of what seemed like a few hundred speed bumps. I learnt a bit about local geography though :wink:

OTOH there have been more than one instance my GPS wanted to provide a state tour instead of just ‘getting there’ and it abused me to no end for refusing to follow instructions. Once in the USA it wanted to go from Toronto to Detroit by crossing the bridge from Canada to the US, doing a U-turn and going back to Canada, and back to the US again. :rofl:


I realise that we use “GPS” as a shortcut for a software mapping and directions system but it needs to be said that the contortions that you describe are not caused by errors in the GPS component of the system. They are a result of a faulty path-finding algorithm or faulty maps in the directions system that employs GPS to find your current location.


I did not imply that. The GPS found the absolute shortest distance, to its credit, as set. Whether it was a good route is a separate topic.

When it saves about 7 metres I think the measurement accuracy might be superb rather than faulty. How did I arrive at 7 metres? The GPS reported distance from A to B, for whatever accuracy that means.


The trouble is that when you laid the blame for inferior routes on the “GPS” (several times) that statement is confusing if the culprit is not the Global Positioning System but another component of the mapping and directions system that you term “GPS”. I don’t doubt you understand quite well what is going on but using the name of a part for the whole is misleading for those who don’t know.


My comments about a state tour and the U turn coming into the US hold as a separate issue to ‘fun using the shortest route’. They are indeed failures of the GPS routing algorithms coupled with their databases. I would not call them inferior routes, I would call them bordering on if not incompetent in cases, but we both understand why that happens.