# Accuracy of GPS V's Car Speedometer



## roker (15 Mar 2013)

Note: I also require (a dashcam) with GPS to record speed on the video.


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## Leo (19 Mar 2013)

GPS recorded speed won't be any good to challenge a speed camera van!


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## roker (19 Mar 2013)

Why not Leo? It's more accurate than the car speedo


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## ajapale (20 Mar 2013)

Leo said:


> GPS recorded speed won't be any good to challenge a speed camera van!





roker said:


> Why not Leo? It's more accurate than the car speedo


Can you point to any evidence to back up the assertion that GPS recorded speed is more accurate than the car speedometer?


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## Guns N Roses (20 Mar 2013)

roker said:


> Why not Leo? It's more accurate than the car speedo


 
There's no way speed determined by GPS will be more accurate than a car speedo. Even the accuracy of the most expensive GPS kit is affected by nearby buildings and trees.


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## Leo (20 Mar 2013)

Car speedos are generally out by up to 10%, and read on the high side. GPS receivers will, on average, give significantly more accurate readings.

But the 'on average' bit is the issue here. GPS systems work by estimating your position, or track points, at preset intervals. For wrist/handheld running/walking GPS systems where high accuracy is required, this can be once per second, for in-car systems, the interval is often longer. 

It then works out the distances between these track points, and calculates your speed between each of these points. They then smooth this data to reduce the impact of erroneous readings, so your in car GPS is really showing you your average speed over the previous number of readings and not an instantaneous reading.

The better handheld trekking devices will warn of poor signal accuracy as caused by tree/building cover. Car systems try to compensate, and be more tolerant of momentary losses by assuming you continue along the same path at the same speed.  
Altitude changes also throw off GPS accuracy, again, the better handheld navigation GPSs that you can load OS mapping to will use the map data to correct for this. The running/cycling GPSs will correct when you upload the data to a computer.

So in the end, your GPS will never give you an accurate instantaeous speed reading that will be in anyway useful for challenging a speed camera. Even my €400 multi-sport GPS says the top speed I recorded running was 125kmph! I wish!


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## roker (20 Mar 2013)

I had communication and a written note from Garmin that a GPS was more accurate than the car, the car is always designed to read faster so that the manufacturer would not get any claims against them for speeding offences, plus the wear on the tyre can make a difference. Every car that I have had my Garmin in, always read about 6 km faster than the Garmin, which surprisingly is quite consistent


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## Guns N Roses (20 Mar 2013)

roker said:


> I had communication and a written note from Garmin that a GPS was more accurate than the car,


 
If I had a Euro for every time a GPS sales rep told me that their product was more accurate than this and that, I'd be a rich man.

It's been my experience that GPS manufacturers will always exaggerate the accuracy of their products.


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## JohnJay (20 Mar 2013)

GPS is only accurate if you are traveling in a straight line. If its a bendy road then it wont be all that accurate.


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## dereko1969 (21 Mar 2013)

The whole point of the OP is for potential defence in a court case against a speed van's estimate of his driving. 

Why not just stick to the speed limit and go with the advice of the speedo in your car - they're always on the high side so by sticking to them you won't risk penalty points/speeding fines, simple no? The money spent on the GPS/Dash Cam could lead you to driving faster than allowed and then ending up with more tickets, so to my mind it's a lose/lose.


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## ajapale (21 Mar 2013)

Speed limits are only one aspect. 

I can see how it might be useful to have an accurate record of the speed together with video evidence of the conditions pertaining at the time.

Fuel consumption would be another factor.


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## tallpaul (21 Mar 2013)

roker said:


> Every car that I have had my Garmin in, always read about 6 km faster than the Garmin, which surprisingly is quite consistent


 
This tallies with my experience. My ye olde satnav (c. 2007 model Garmin) is consistently 5km less than the speedometers of the three cars it has been used in.


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## roker (21 Mar 2013)

dereko1969 said:


> Why not just stick to the speed limit and go with the advice of the speedo in your car - they're always on the high side so by sticking to them you won't risk penalty points/speeding fines, simple no? The money spent on the GPS/Dash Cam could lead you to driving faster than allowed and then ending up with more tickets, so to my mind it's a lose/lose.


 
I am not suggesting that I should go against the speed limits, I am suggesting that the speed cameras are flawed and could give and incorrect speed and summons, If I am speeding I have no excuse


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## Guns N Roses (22 Mar 2013)

roker said:


> I am suggesting that the speed cameras are flawed and could give and incorrect speed and summons,



What reason do you have to suggest that the speed cameras are flawed? Most of them are brand new and I would imagine that they are calibrated regularly in order that their readings stand up in court.


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## TarfHead (22 Mar 2013)

roker said:


> I am not suggesting that I should go against the speed limits, I am suggesting that the speed cameras are flawed and could give and incorrect speed and summons, If I am speeding I have no excuse


 
I think the prospect of the Gardai conceding that their equipment is defective or of a Court upholding a "defendant's" evidence over the Gardai, is remote if not impossible. Think of the precedent it would set and the implications of that.

If the Garda equipment is wrong, it's the only measure against which your speed can be judged and any attempt to provide otherwise is, IMHO, pointless.

What's next ? Getting different equipment to measure blook alcohol levels ?


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## serotoninsid (22 Mar 2013)

Insofar as I understand it, you have a right to request the calibration sheet for a hand-held speed gun.  Presumably if you went to court, you could request the same for either a fixed camera or the mobile gosafe cameras?  

If the calibration is uptodate, I would imagine that's the end of your challenge (regardless of what stats you present by way of a gps device)?


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## roker (22 Mar 2013)

*The great speed gun scandal*

Speed demon? Tests show gun is faulty
This is the heart-stopping moment every motorist dreads. As you drive along the road, a police officer points a laser speed gun towards you.
Glancing at the dashboard, you breathe a sigh of relief: the speedometer reveals that your car is travelling below the 30mph limit. But a month later, a letter drops through your door. You face a fine for speeding and penalty points on your licence.
It is claimed that you were driving at 41mph - not 28mph. Can that high speed really be true? Staggeringly, the answer may be no.
Motorists accused of driving too fast on Britain's roads insist the real culprit is a laser speed gun officially approved by the Home Office and used by almost every police authority in the country. For the Mail has discovered that the LTI 20.20.gun is seriously flawed.
In our tests, it wrongly recorded a wall as travelling at 44mph, an empty road scored 33mph, a parked car was clocked as doing 22mph and a bicycle (in reality being ridden at 5mph) rocketed along at an impossible 66mph.
Imported from America, the LTI 20.20. is used in nearly 3,500 mobile speed units hidden in police vans or cars and mounted on motorbikes.
Speed traps - nearly half of which now use laser gun technology - reap more than £100 million each year in fines. This is shared between the police, the Highways Agency, the courts, the Home Office and local authorities.
Ironically, some of the huge sum is used to pay for even more police speed reinforcement teams relying on exactly the same laser speed gun at the centre of the Mail's investigation.
*Rigorous tests*
We subjected the speed gun to rigorous tests. Alarmingly, we discovered it was prone to wildly wide-of-the-mark readings, even when set up according to the police's own guidelines and the manufacturer's handbook.
In other tests, we found the equipment was measuring the speed of overtaking cars instead of the one being targeted.
Today, the Mail can expose the scandal of a speed enforcement industry in which the collection of fines is considered paramount - whatever the consequences for innocent drivers caught in police traps by faulty readings.
In the past nine years, an extraordinary one-in-five drivers has been fined for speeding, despite many protesting their innocence.
Lawyers we spoke to say motorists are now rebelling by refusing to pay fines and fighting their cases through the courts.
One voicing concern is Barry Culshaw, a Southampton solicitor currently acting for 15 drivers nationwide. "They complain of huge errors," he says. "Drivers say they were within the speed limit and yet the LTI 20.20. recorded them doing excessive speed."
Another disquieting discovery is that vital video film - often taken at a speed-trap site for use as secondary evidence - is often mysteriously withheld from motorists by the Crown Prosecution Service.
On at least ten occasions the Crown has suddenly dropped the case against a motorist when ordered by a judge to hand over the telling footage.
Michael Morgan, who runs a British website collating complaints against laser speed guns, said: "The authorities often wriggle rather than release the video, which would expose the laser gun to scrutiny in a court of law. No doubt they fear the enormous consequences, including a clamour for fine refunds and compensation over the loss of licences or even livelihoods."
*Expert witnesses compromised*
Alarmingly, the Mail can reveal, too, that the main expert witness used by the CPS to convict motorists in such cases - a former police officer named Frank Garratt - also makes his living as boss of the company importing the devices into Britain. Perhaps not surprisingly, Mr Garratt, a millionaire, told the Mail the LTI 20.20. works perfectly well.
One of the gun's toughest critics is Dr Michael Clark, Europe's leading expert on laser technology. He is a former company director of a British firm making laser detection equipment for traffic lights and car parks.
Dr Clark was clocked, apparently speeding, by a laser gun three years ago. He fought his case through the courts, proving he was travelling below the limit. He has acted as an expert witness on behalf of many motorists since.
"I was drawn into this controversy because I know about laser science. I do not rely on my court appearances or the speed enforcement industry to make a living," he told the Mail when we asked him to help - without payment - in our experiments.
Dr Clark says that the gun is defective because its wide beam can easily pick up the wrong vehicle. Furthermore, if the device is not held firmly on the target - and this is a difficult task - it can produce an erroneous speed result by "slippage".
Reflections from road signs and from other cars - even one stationary on the kerbside - can also make the laser gun misinterpret the truth.
*THIS IS AN ABRIDGED VERSION OF THE STORY THAT APPEARED IN THE DAILY MAIL*.

Plus don't forget the tractor in Ireland clocked doing about 80mph




*.*


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## RichInSpirit (22 Mar 2013)

I had my gearbox changed once and now the speedometer is seriously reading wrong. 

On the Motorway 90MPH on the speedometer corresponds to 75MPH on the garmin.
And I believe the garmin because it seems to be very accurate with journey times. Only out by a minute or 2 on most journeys.


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## Leo (22 Mar 2013)

serotoninsid said:


> Insofar as I understand it, you have a right to request the calibration sheet for a hand-held speed gun. Presumably if you went to court, you could request the same for either a fixed camera or the mobile gosafe cameras?
> 
> If the calibration is uptodate, I would imagine that's the end of your challenge (regardless of what stats you present by way of a gps device)?


 
I believe the legislation here is worded such that no evidence of calibration is required. Essentially, it's worded so that there is no valid argument, and no amount of external evidence will make a difference.

Roker, if you want to challenge or change the system, it's the politicians you need to focus your energy on.


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## ajapale (22 Mar 2013)

roker said:


> *The great speed gun scandal*
> ............
> *THIS IS AN ABRIDGED VERSION OF THE STORY THAT APPEARED IN THE DAILY MAIL*.





ajapale said:


> But the Daily Mail article in your original post  specifically refers to one specific model of laser gun - LTI 20.20.
> 
> 
> The Daily Mail story appears to date from 2005.


 See: Speed Camera Reliability

Roker, you will need better evidence than a breathless Daily Mail Report from eight years ago to convince me!
aj


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## roker (22 Mar 2013)

The technology is the same no matter what the maker is. There are strict rules for using the camera, the Garda are not going to admit they did not follow procedures. They will also not produce evidence of calibration, I have tried, they may as well have a dummy camera and guess the readings


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