Beat speeding fines with GPS

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Map Monkey
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Re: Beat speeding fines with GPS

Post by Map Monkey » 23 June 10 11:42 pm

Wow, a 10-second tracklog is accepted as precise data. :shock: Might guess that he slowed down within that timeframe. :-k

mm

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Richary
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Re: Beat speeding fines with GPS

Post by Richary » 24 June 10 12:31 am

It's interesting, in the higher gears my car speedo under reads by about 5% according to the GPS. That makes sense as I have fitted larger than standard tyres to it that are about 5% bigger. In the lower gears the speedo under reads.

Every hire car I have ever had the speedo over reads according to the GPS when I have taken it along. A strategy by the hire car companies to stop them getting tickets they have to process and forward on? And over the last 4 years I have probably had 20 different hire cars on work trips.

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Coffee and Cache
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Re: Beat speeding fines with GPS

Post by Coffee and Cache » 24 June 10 1:00 am

tronador wrote:
J_&_J wrote:I've noticed while caching in both our vehicles (Holden Viva & Toyota Hilux) that the GPSr speed is always about 3-4 kph lower than the speedo reading on the car when travelling at roughly 110kph. I have to do a touch under 115kph in my Hilux for the GPS60 to register 110kph. I've wondered about the legal aspect of this, and which is the more accurate.
Yes I agree. If I set my cruise control to 110km/h according to my Tom Tom, this is vastly different to what the speedometer says, which is usually close to 115-120km/h. Which one is correct??? If I see a fixed camera I usually go by the speedo not the GPS to be safe. Has any official test been done to determine whether the GPS is actually more accurate than the speedo/
Car speedo's in recent times usually read high, because ADR regulations prohibit them from indicating a speed lower than the actual speed, so to allow for the considerable error, they have to be set to read slightly high.

From ADR regs

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

There is no question that the GPSr is more accurate than a car speedo. GPSrs that read 1000's of kmh standing still have a big problem. If a GPSr has a good satellite fix, there is no excuse for that. Of course, there is no guarantee of quality either - hardware or firmware, in the sort of receivers used in geocaching and other outdoor activities.

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Yurt
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Re: Beat speeding fines with GPS

Post by Yurt » 24 June 10 6:43 pm

Richary wrote:It's interesting, in the higher gears my car speedo under reads by about 5% according to the GPS. That makes sense as I have fitted larger than standard tyres to it that are about 5% bigger. In the lower gears the speedo under reads.

Every hire car I have ever had the speedo over reads according to the GPS when I have taken it along. A strategy by the hire car companies to stop them getting tickets they have to process and forward on? And over the last 4 years I have probably had 20 different hire cars on work trips.
I've found the same with cars in five states, they always read about 2-4 km/h higher than the GPS. I would think that the GPS speed is more accurate over the short (but not too short) term. Being tracked by many satellites in the open should be more accurate than a mechanical gizmo in a car?

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tronador
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Re: Beat speeding fines with GPS

Post by tronador » 24 June 10 10:08 pm

Coffee and Cache wrote:
tronador wrote:
J_&_J wrote:I've noticed while caching in both our vehicles (Holden Viva & Toyota Hilux) that the GPSr speed is always about 3-4 kph lower than the speedo reading on the car when travelling at roughly 110kph. I have to do a touch under 115kph in my Hilux for the GPS60 to register 110kph. I've wondered about the legal aspect of this, and which is the more accurate.
Yes I agree. If I set my cruise control to 110km/h according to my Tom Tom, this is vastly different to what the speedometer says, which is usually close to 115-120km/h. Which one is correct??? If I see a fixed camera I usually go by the speedo not the GPS to be safe. Has any official test been done to determine whether the GPS is actually more accurate than the speedo/
Car speedo's in recent times usually read high, because ADR regulations prohibit them from indicating a speed lower than the actual speed, so to allow for the considerable error, they have to be set to read slightly high.

From ADR regs

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

There is no question that the GPSr is more accurate than a car speedo. GPSrs that read 1000's of kmh standing still have a big problem. If a GPSr has a good satellite fix, there is no excuse for that. Of course, there is no guarantee of quality either - hardware or firmware, in the sort of receivers used in geocaching and other outdoor activities.
Thanks for this insight, but aren't the manufacturers of the car speedo misleading the driver. Wouldn't it be better to have the correct speed not a higher speed than being driven. This is irritating. As a driver i want to know the correct speed. This is just as irritating as people who set their watch faster than the actual time. :evil:

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Coffee and Cache
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Re: Beat speeding fines with GPS

Post by Coffee and Cache » 24 June 10 10:29 pm

tronador wrote:
Thanks for this insight, but aren't the manufacturers of the car speedo misleading the driver. Wouldn't it be better to have the correct speed not a higher speed than being driven. This is irritating. As a driver i want to know the correct speed. This is just as irritating as people who set their watch faster than the actual time. :evil:
Well, I agree, but I suspect the people who decide these things don't want people arguing that their speedo said they were not speeding when they were, even if by a trivial amount. Allows the powers that be to dismiss the speedo may be in error argument - not that I have any legal training of course. I had a VL Holden that read under the true speed by about 5km/h at 100km/h. It was quite irritating knowing that I really needed to keep the needle under the 100 mark to stay legal. Newer cars of course are not supposed to be like that, but who knows how many correctly comply with the ADR reg.

Incidentally, I don't think the gear should make any difference to the speedo, the take-off is from the tail-shaft or equivalent, not the motor.

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tronador
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Re: Beat speeding fines with GPS

Post by tronador » 24 June 10 10:35 pm

Coffee and Cache wrote: Incidentally, I don't think the gear should make any difference to the speedo, the take-off is from the tail-shaft or equivalent, not the motor.
:shock: Huh??? Sorry but I have no idea what this means.

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Richary
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Re: Beat speeding fines with GPS

Post by Richary » 24 June 10 10:52 pm

Coffee and Cache wrote:Incidentally, I don't think the gear should make any difference to the speedo, the take-off is from the tail-shaft or equivalent, not the motor.
tronador - I think they were replying to my comment.

I agree, and was quite surprised when I noticed it, but I can only report what I see on the speedo vs GPS. It's a 4WD with Low Range capability so the speedo has to read off the tail shaft speed to be any sort of accurate. In 4th and 5th the car speedo under reads as expected due to the bigger tyres, but in lower gears it appears to read high.

I might have to do some more scientific tests, using both the TomTom and Garmin handheld to register speed while going at a constant speed in different gears to compare the speedo. Assuming I find somewhere I can sit on say 20kph in 2nd for a few hundred metres without annoying anyone too much!

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Coffee and Cache
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Re: Beat speeding fines with GPS

Post by Coffee and Cache » 24 June 10 11:22 pm

Richary wrote:
Coffee and Cache wrote:Incidentally, I don't think the gear should make any difference to the speedo, the take-off is from the tail-shaft or equivalent, not the motor.
tronador - I think they were replying to my comment.

I agree, and was quite surprised when I noticed it, but I can only report what I see on the speedo vs GPS. It's a 4WD with Low Range capability so the speedo has to read off the tail shaft speed to be any sort of accurate. In 4th and 5th the car speedo under reads as expected due to the bigger tyres, but in lower gears it appears to read high.

I might have to do some more scientific tests, using both the TomTom and Garmin handheld to register speed while going at a constant speed in different gears to compare the speedo. Assuming I find somewhere I can sit on say 20kph in 2nd for a few hundred metres without annoying anyone too much!
Err - yes. Sorry about that. Trying to kill two birds with one stone so to speak.

I think your assuming the speedo is linear over it's range. It's quite possible it reads low at some speeds and high at others. These might correspond to different gears. You'd have to stay in the same gear over a wide speed range to check.

The issue with tyre pressure is also quite interesting - well at least to some. There was much heated debate around the lunch table at work a few years back about the effect of tyre pressure on speedo readings. One argument goes that at low pressures the tyre diameter is less, and of course it rotates faster at any given speed, so naturally the speedo will read high when the tyre is flat. On the other hand, there is an argument that the circumference of the tyre stays the same no matter what state of inflation, it just deforms ahead and behind of where it contacts the road, and thus the wheel rotates at the same speed and the speedo is not effected. If any one knows the correct answer, I'd love to know.

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Re: Beat speeding fines with GPS

Post by J_&_J » 25 June 10 12:10 am

To go slightly off-topic....

I used to own a 1978 Holden ute that had the standard Aussie 4 speed gearbox and what was known as a "highway" differential. The car had appalling acceleration but could hit 110kph in 3rd gear. The speedometer was driven from the gearbox, so needless to say I never had an accurate indication of my true speed, I just knew I had to keep the needle a fair bit below the mark to be legal.

How I never got pinged I'll never know!

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Re: Beat speeding fines with GPS

Post by Bewilderbeest » 25 June 10 9:29 am

squalid wrote:Last time I got a fine, I thought "can't remember going down that road", so checked my GPS tracks, and Yes I did go down that road AND I was speeding!
Last time I got a speeding fine, GPS hadnt been invented \:D/

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Re: Beat speeding fines with GPS

Post by caughtatwork » 25 June 10 9:30 am

How did you get a speeding fine on a horse and buggy ?

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Re: Beat speeding fines with GPS

Post by Bewilderbeest » 25 June 10 10:27 am

The speed limit was a lot lower then...if the centurion had to run to catch you, then you were going too fast!

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Re: Beat speeding fines with GPS

Post by Hoojar » 25 June 10 10:42 am

When I bought my '07 Forester I noticed straight away that the speedo under reads by 10%. No matter what speed I am doing, the Speedo was 10% under the GPSr. I even tried setting up multiple GPSrs to test, and the results were conclusive.

And so on the first service I requested that my Speedo be re-calibrated. This request was outright refused. Instead I was handed a three page booklet from Subaru Australia explaining in depth about how the Australian Standard requires speedos to be accurate to within a 10% tolerance, and so for the sake of drivers not receiving speeding fines, they deliberately calibrate to read 10% under.

The thing that most amazes me is that I understand in Victoria they have a 0% tolerance for speeding (and will fine you for being 1km/h over the limit) however the Australian Standard seems to infer that it is entirely possible that you can be 10% over the speed limit and have no way of knowing so (unless you have a GPSr).

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Re: Beat speeding fines with GPS

Post by FarmerFrentzen » 25 June 10 10:55 am

Hoojar wrote:When I bought my '07 Forester I noticed straight away that the speedo under reads by 10%. No matter what speed I am doing, the Speedo was 10% under the GPSr. I even tried setting up multiple GPSrs to test, and the results were conclusive.

And so on the first service I requested that my Speedo be re-calibrated. This request was outright refused. Instead I was handed a three page booklet from Subaru Australia explaining in depth about how the Australian Standard requires speedos to be accurate to within a 10% tolerance, and so for the sake of drivers not receiving speeding fines, they deliberately calibrate to read 10% under.

The thing that most amazes me is that I understand in Victoria they have a 0% tolerance for speeding (and will fine you for being 1km/h over the limit) however the Australian Standard seems to infer that it is entirely possible that you can be 10% over the speed limit and have no way of knowing so (unless you have a GPSr).
My 2007 Subaru Forester XT speedo also reads exactly 10% higher at all speeds in comparison with my GPSr and my other cars. I had assumed it may have had something to do with the wheels and tyres on the XT being different to the standard model and that the speedo is calibrated based on the standard model only. Perhaps it is intentionally 10% lower as Hoojar was advised by Subaru.

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