Speedo Inaccurate

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Originally Posted by Maverick

GPS can be used as evidence in court if you record the HDOP and depending on what the car is doing at the time has a decent update interval (5Hz is a good interval). If you have a video with this device providing the overlay you won't have any problems with it being accepted in court as long as the HDOP is <1.00 (or not far from it, the video helps as it shows what the movement is like and can be used with the GPS time to verify the GPS accuracy if required by using points of reference on the road.

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Sorry, i was misremembering. There have been a few cases in Victoria where GPS evidence was used in court, and the Victoria Police were trying to get GPS evidence declared non admissable It seems they didn't succeed. This is a good thing.

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Originally Posted by Maverick

GPS tracking systems with no HDOP and with logs that are updated only a few times a minute have been used in court before on both sides and been accepted so by using something like the system above it's very unlikely you would have any problems.

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That's amazing. If all you had was a log on a flash card or the like, what guarantee is there that the data hasn't been manipulated. see http://www.gmat.unsw.edu.au/snap/pub...n&lim2008d.pdf

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Originally Posted by Maverick

The inaccuracy has little/nothing to do with changing satellites, the GPS connects to as many satellites as it can but it has everything to do with the location of the satellites. The DOP will be high (which is poor) if they're too close together (this can occur in cities for example) whereas the best DOP is if the satellites are far apart which gives a low DOP.

If you have a high DOP value you won't have a 3D fix so the GPS won't know if you're doing up or down a hill which affects the accuracy even further.

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I can understand that. GPS is basically triangulation, so the further apart your reference points (the satellites in this case) the better you know the position. Would the error vary between subsequent position fixes sufficiently to put speed measurement out? After all at 5hz, a car at 30 m/s ( 108 km/hr) travels 6m between fixes, so the error in subsequent readings would be similar.

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Originally Posted by cme2c

Sorry, i was misremembering. There have been a few cases in Victoria where GPS evidence was used in court, and the Victoria Police were trying to get GPS evidence declared non admissable It seems they didn't succeed. This is a good thing.

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Some of the data they're accepting though could be easily altered and this is what they need to look out for. I guess the more serious the case or the more it occurs the more likely it is to be scrutinised.

The device I linked to above I use and it records on high definition video whenever the cars ignition is on plus another GPS unit that logs to a card and can be used for remote kill and tracking (about to be fitted).

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That's amazing. If all you had was a log on a flash card or the like, what guarantee is there that the data hasn't been manipulated. see http://www.gmat.unsw.edu.au/snap/pub...n&lim2008d.pdf

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I'm pretty sure the one I was thinking about is the one mentioned in that PDF, this guy had a gps that sticks onto the windscreen and logs data to a memory card inside the unit.

"Recently, a motorist in New South Wales (NSW) was fined $203 for
allegedly driving at 85 km/h in a 60 km/h zone. The motorist challenged the fine in court and presented data
from his on-board GPS navigator which showed that he was mostly travelling at a speed of 57 km/h on that
particular stretch of the road, which was also corroborated by a GPS expert in court. The motorist challenged the
accuracy of the hand-held radar guns questioning how rigorously these guns were calibrated each year. The
traffic officials conceded in court that they had not taken the readings on their radar guns for the required length
of time and had simultaneously relied on their experience and visual estimates. The fines were overturned in the
district court setting a precedent for the admissibility of GPS evidence in NSW."

I saw two possible issues with that (and I don't know the refresh rate but I'm guess it was may writing to the card every 5 or 10 or 30 seconds) and that was the refresh rate could have meant he could have accelerated than braked and as you said that he could have altered the data as from the logs that I saw it didn't have a lot of detail (if it had all the details like the satellite positions etc it would be incredibly hard to fake).

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I can understand that. GPS is basically triangulation, so the further apart your reference points (the satellites in this case) the better you know the position. Would the error vary between subsequent position fixes sufficiently to put speed measurement out? After all at 5hz, a car at 30 m/s ( 108 km/hr) travels 6m between fixes, so the error in subsequent readings would be similar.

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Not sure what you mean, the HDOP is calculated constantly (5Hz) so you can see if the error has changed (ie going under a bridge will caused the HDOP to jump for that one or two updates) so any error is able to be seen. I generally find the HDOP is around or under 1.0 for just about everywhere.

1Hz updates can be touch and go for court from what I've read from US cases as it can be argued the speed can change in that 1 second by a reasonable amount (which is true) so with 5Hz that should cover any issues with fast alterations in speed (this is good if you accelerate hard from a standstill as with a 1Hz unit it jumps in large increments of around 25kph whereas the 5Hz is in 5kph jumps).

If you don't have the HDOP on the video or logs however it can be argued that it's not accurate. HDOP works off the number of satellites visible and their location in the sky (can't remember if there is anything else it uses to calculate?) and is an accurate measure of the validity of the data. Even if you don't get under 1 there are bandings like 1.01-5 is very accurate and so on up to I think 20-50 which is basically useless as the error is enormous.

A question for the people that are happy with the speedo error, do drive to the speed limit as it is indicated on the speedo, or do you drive to an indicated speedo reading that you believe is closer to the true speed? For example, in an 80 zone, do you drive at an indicated 80 or an indicated 86?

i work at wodonga prestige VW.. i wasnt meaning that GPS speed cant be used as evidence in court, theres a formula used by manufactures to calibrate the speedo's and to calculate it you need to know your "true speed" and your "indicated speed" ( shown on the speedo) and you cant use gps to find true speed as it seen to not be 100% accurate

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Originally Posted by Topend

A question for the people that are happy with the speedo error, do drive to the speed limit as it is indicated on the speedo, or do you drive to an indicated speedo reading that you believe is closer to the true speed? For example, in an 80 zone, do you drive at an indicated 80 or an indicated 86?

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I drive at the corrected speed that I have calculated to account for the speedo error. So I drive at 106 indicated in a 100 zone and 85 in a 80 zones.

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Originally Posted by Topend

A question for the people that are happy with the speedo error, do drive to the speed limit as it is indicated on the speedo, or do you drive to an indicated speedo reading that you believe is closer to the true speed? For example, in an 80 zone, do you drive at an indicated 80 or an indicated 86?

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I drive to the error most of the time . I set the cruise control to the error. Keeps the mind active.

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Originally Posted by brodzta_GTI

i work at wodonga prestige VW.. i wasnt meaning that GPS speed cant be used as evidence in court, theres a formula used by manufactures to calibrate the speedo's and to calculate it you need to know your "true speed" and your "indicated speed" ( shown on the speedo) and you cant use gps to find true speed as it seen to not be 100% accurate

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Thanks for that.

For those of you interested in the Speedo test results I posted yesterday I have now had the chance to test the remaining distance multiplier 8. If you recall it is the only one that will skew speeds downwards from standard on my MK6. As previously mentioned it reduces the speedo overread slightly from 106 kph to 104kph. Can now confirm it does indeed skew the odometer by -1% as I had expected.
So in summary, just as the makers of VCDS, Ross Tech said, "playing with distance mutliplier will skew all three speeds". On a MK6, Setting it away from VWs intended setting will induce an error in your odometer and will not go close to removing the error in your speedo. So leave it alone I reckon.

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Originally Posted by logger

For those of you interested in the Speedo test results I posted yesterday I have now had the chance to test the remaining distance multiplier 8. If you recall it is the only one that will skew speeds downwards from standard on my MK6. As previously mentioned it reduces the speedo overread slightly from 106 kph to 104kph. Can now confirm it does indeed skew the odometer by -1% as I had expected.
So in summary, just as the makers of VCDS, Ross Tech said, "playing with distance mutliplier will skew all three speeds". On a MK6, Setting it away from VWs intended setting will induce an error in your odometer and will not go close to removing the error in your speedo. So leave it alone I reckon.

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Got it, thanks.

I've just read this thread through, and to be honest I'm disgusted with VW.

What VW is basicaly saying is, we don't think you should be alowed to make discisions for yourself.

I have a Patrol which was pretty much on the money from factory. I put bigger rubber onto it and went to the trouble of finding the right gear to put into the gearbox (oldschool gear drive) to correct the 12% error the rubber introduced.

Now I find out VW expect me to buy a $50K vehicle which has a fault built in?

I haven't ordered my GTI yet was going to do it next week - but to be honest this will piss me off everytime I drive it - I'm seriously wondering if I shouldn't go to the WRX which was my second choice at least for all of its faults Subaru doesn't tell you you are too dumb to make your own descisions.


Seriously not good enough VW - I agree with the link provided and the comment by Toyota, why should a car company be directed by ill thought out idiology - which quite clearly doesn't work as everyone who answered stated that they just took the error into account.


Thanks for the comment guys and the info on this forum - I'll go chat to my dealer I think - while it might seem insignificant to some this could well be a deal breaker for me, not so much for the action, but for the attitude towards its customers.