speedometer inaccuracies

[Ootohere]

Re #73. the measurements are not the Tread on the ground width unless that is the widest point, it is the width of the tyre, and with some tyres that can be the Rim Protector or the sidewall, or it could be the tread.

 

Skoda / VW where giving permission for dealers to fit Pirellis when cars with Dunlop or Continental had Pulling to the left issues, 

the stronger sidewalls & less tread on the road for the same size tyres and the sometimes skittery performance hid the misalignment from the factory and corrected nothing.

[Urrell]

Don't forget any GPS speed reading is only accurate when the car is on a flat level road, and any gradient will make the reading inaccurate, as the "GPS" only ever sees the road your on as flat and level, so maybe not as accurate a method of measurement as you may think, best used when calculating a speed as an average over many driven miles.

GPS does know exactly your altitude as you climb and descend in height.

Even the Amundsen II fitted in the Yeti has a running display of your altitude as well as position.

[fordfan]

 

I keep a small aide-memoire printed out on an index card and tucked under the strap on the driver's sun visor.  Any time I need reminding - especially when I've just changed tyres - I can just flip the sun visor down and there they are.  The kph speeds are as indicated on the MFD.  These figures have been checked vs GPS.  The red percentages show the error between indicated and actual:

 

225/50R17

70 mph = 119 kph 5.6%

60 mph = 102 kph 5.6%

50 mph = 86 kph 6.8%

40 mph = 69 kph 7.2%

30 mph = 53 kph 9.8%

 

205/55R16

70 mph = 125 kph 10.9%

60 mph = 108 kph 11.8%

50 mph = 90 kph 11.8%

40 mph = 72 kph 11.8%

30 mph = 55 kph 13.9%

20 mph = 38 kph 18.0%

 

This suggests that my car's speedo is technically illegal when I have my winter tyres fitted.  Hmm...

 

(I have recently installed a NextBase dashcam which displays speed measured by GPS, so the card is now basically redundant.)

 

We had an Octavia Diesel in which had been fitted with a gearbox from a petrol model.  A quick spin up the road and a play with VCDS to adjust the speedometer constant and it read within the permissible error limit.

[ejstubbs]

GPS does know exactly your altitude as you climb and descend in height.

 

"Exactly" is overstating it rather.  Even the positional (lat/long) fix is not 'exact' and, as Brijo noted, the altitude is almost always less accurate due to limitations inherent in the system.

 

Whether or not the displayed speed is calculated taking altitude changes into account is by no means certain - and can easily vary between devices.  My observation this afternoon - where the GPS speed remained the same while the MFD speed (which I assume is based on some kind of sensor on the transmission) chnaged - suggests that, in the case of my dashcam at any rate, it isn't.

Edited 26 November, 2016 by ejstubbs

[Ryeman]

Mine never varies more than 1kph ; floating between speedo-4 and -5 , and in any case you might be doing 99.5 or 100.5 actual.

Assuming the speed gun is accurate, we can get away with 107 indicated as a genuine 103 - 2 would be an alleged 101 which they wouldn't prosecute.

Of course, if the gun was over reading by 2 (the error allowed for being + or - 2) you might be unlucky and get pinged for the full 103

That's in Victoria of course

Are you feeling lucky?.

[muddyjim]

 

 

225/50R17

70 mph = 119 kph 5.6%

60 mph = 102 kph 5.6%

50 mph = 86 kph 6.8%

40 mph = 69 kph 7.2%

30 mph = 53 kph 9.8%

 

205/55R16

70 mph = 125 kph 10.9%

60 mph = 108 kph 11.8%

50 mph = 90 kph 11.8%

40 mph = 72 kph 11.8%

30 mph = 55 kph 13.9%

20 mph = 38 kph 18.0%

 

This suggests that my car's speedo is technically illegal when I have my winter tyres fitted.  Hmm...

 

 

AIUI the actual UK requirement is that between 25mph and 70mph the speedo should read no more than 10% + 6.25mph of the true speed -  I think you are well within this...

 

http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

 

2.  For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed.

 

3.  For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed—

V/10 + 6.25 mph where V = the true speed of the vehicle in mph. 

 

Jim

Edited 26 November, 2016 by muddyjim

[ejstubbs]

Well, that's interesting.  I'd always understood it to be a straight 10%, but the additional fixed allowance does indeed exist.  The situation still seems to be slightly confused though.  UNECE regulation 39 and the corresponding EC directives say 10% + 4kmph.  However, the UNECE regulation allows more leeway - 10% + 6kmph - during Conformity of Production Audits on mass produced vehicles following type approval.  How the C&R regs ended up with 10% + 6.25mph (10kmph) is not exactly clear.

[Urrell]

There has been lots of talk and in the press that some police forces will be prosecuting for anything over the limit, no 10% and no leeway.

[muddyjim]

I guess it's because - like so many regulations - the EU have tighter restrictions when compared to the UK?

 

Jim

 

How the C&R regs ended up with 10% + 6.25mph (10kmph) is not exactly clear.

[muddyjim]

There has been lots of talk and in the press that some police forces will be prosecuting for anything over the limit, no 10% and no leeway.

I think that has often been the case. Speedo's should never show less than the true speed - so, providing you never drive with a speedo reading higher than the speed limit you will never exceed the speed limit.

 

Jim

[Ootohere]

After a 6 month pilot scheme in Scotland for formal warnings for those just over the speed limit the new scheme was introduced this year.

http://scotland.police.uk/whats-happening/news/2016/january/new-recorded-police-warning-scheme-introduced

http://www.driving.co.uk/news/just-1mph-too-fast-and-youre-nicked-new-zero-tolerance-approach-to-speeding

Edited 27 November, 2016 by Offski

[ejstubbs]

There has been lots of talk and in the press that some police forces will be prosecuting for anything over the limit, no 10% and no leeway.

 

After a 6 month pilot scheme in Scotland for formal warnings for those just over the speed limit the new scheme was introduced this year.

http://scotland.police.uk/whats-happening/news/2016/january/new-recorded-police-warning-scheme-introduced

http://driving.co.uk/news/just-1mph-too-fast-and-your-nicked-new-zero-tolerance-approach-to-speeding

 

The 'leeway' being discussed previously was the variation allowed in speedo accuracy, nothing to do with leeway allowed on actual speeding offences.

 

As Muddyjim pointed out, speedos are never supposed to read high, so provided you use your speedo to observe the speed limit, you should be fine in that regard.

 

Conversely, drivers who have taken the trouble to discover the actual degree of inaccuracy in their speedos can still be perfectly within the law in overtaking someone driving to what their speedo shows, provided they do so safely.  The 'amateur motorway police' who justify lane-hogging on the basis that "I'm doing 70, you know" because that's what their speedo says can be quite frustrating.  I wonder how many of them realise that they could be doing less than 60mph and their speedo still be legal?  Could explain some behaviour I've seen from time to time.

Edited 27 November, 2016 by ejstubbs

[Urrell]

The 'leeway' being discussed previously was the variation allowed in speedo accuracy, nothing to do with leeway allowed on actual speeding offences.

Usually most police forces go on 10% plus 2 mph (35 in a 30) but these are only guidelines laid down by the Association of Chief Police Officers, or ACPO as they are usually known.

In reality some have cameras set to lower than the guidelines and they can and will prosecute for 1 mph over. the limit is the limit and that is that with some of them.

[Ootohere]

ejstubbs,

i posted the links in responce to post #83. I have followed the thread since post #1, so know what this epic is about.

 

Just call it what you want, but Police Scotland made it clear that ACPO Guidelines was not what they were following, they were going for Education & Safety.

(& they like being able to stop people because the Lower Drink Driving Limit in Scotland was not having an significant higher catch rate, so stopping just over the speed limit had them speaking to more drivers, and catching the breath, and it was handy for other Causes to stop vehicles.

http://heraldscotland.com/news/13415834.Police_get_new_powers_as_they_set_sights_on_drivers_just_over_speed_limit

Edited 27 November, 2016 by Offski

[logiclee]

Checking our two Yeti's which are identical apart from one has Pirelli's and the other Goodyear's. 

 

The Goodyear's appear to have smaller circumference as the speedo error is higher than the Pirelli's.

 

Lee

[KenONeill]

GPS does know exactly your altitude as you climb and descend in height.

Even the Amundsen II fitted in the Yeti has a running display of your altitude as well as position.

Which is nice, as far as it goes. Unfortunately, that isn't always very far. I was once checking the calibration of a handheld Garmin using a triangulation pillar which happens to be just outside my work as the reference. I got a sustained sink rate of 30m/s for several seconds at one point, and that's going some for a 300 feet high hill made of Lewisian Gneiss.

[Urrell]

Are you saying the Yeti or GPS in general is not accurate?
Mine always reads the same when in the same place and does not move when I'm stationary.
Quite a few bulldozers are fitted with GPS levelling gear that is pretty accurate and they measured Scafell Pike on TV this week with GPS and that was to within a millimetre.
The Yeti only shows to the nearest meter.

[Ryeman]

The modern tractor ploughs a very straight furrow and are all but fully automated.

Not sure, but the U.S. military controlled who could access their highest standard of accuracy, but now the required aviation standard seems to have made the standard available to all, probably because a civil facility would have been made it universally available in any case.....that's probably the situation now

[Urrell]

This shows how tractors have come on using GPS.
The first clip is from 6 years ago so it's not new technology.

 

[Ryeman]

Oh no

It's been around for years and accuracy seems to be down to a few centimetres.

Aircraft could fly without pilots if it didn't freak out the passengers.

[KenONeill]

Are you saying the Yeti or GPS in general is not accurate?

Mine always reads the same when in the same place and does not move when I'm stationary.

Quite a few bulldozers are fitted with GPS levelling gear that is pretty accurate and they measured Scafell Pike on TV this week with GPS and that was to within a millimetre.

The Yeti only shows to the nearest meter.

Hint in "handheld Garmin" I thought?

 

Not all GPS are created equal (in aerials or software), and that's before we even look at whether you have a vanilla GPS or a differential GPS (DGPS).

[ejstubbs]

Not all GPS are created equal (in aerials or software), and that's before we even look at whether you have a vanilla GPS or a differential GPS (DGPS).

 

Indeed.

 

This is quite a useful web site: http://www.gps.gov/systems/gps/performance/accuracy/

 

...the GPS signal in space will provide a "worst case" pseudorange accuracy of 7.8 meters at a 95% confidence level. (This is not the same as user accuracy; pseudorange is the distance from a GPS satellite to a receiver.)

 

The actual accuracy users attain depends on factors outside the government's control, including atmospheric effects, sky blockage, and receiver quality. Real-world data from the FAA show that their high-quality GPS SPS receivers provide better than 3.5 meter horizontal accuracy.

 

Neither your handheld GPS nor the one built in to your satnav is as accurate as an aeronautical GPS.

 

The GPS measurement of the height of Scafell Pike would have used a surveying GPS - again, a different beast to your common or garden consumer GPS device.  Those things can cost as much as an entry-level Yeti.

 

I'm sure the GPS-steered tractors also use something rather more sophisticated than a consumer-level GPS chip.  The additional cost for GPS guidance at the level of accuracy required for that job is affordable in comparison to the overall cost of the vehicle.  They're not particularly "agricultural" vehicles these days - not in the primitive, fix-it-with-a-hammer sense of the word, anyway!

 

Are you saying the Yeti or GPS in general is not accurate?

Mine always reads the same when in the same place and does not move when I'm stationary.

 

The location displayed on the map may not change, but that's likely because the map resolution is lower than the margin of error in the GPS fix.

 

It's not at all uncommon for a consumer-level GPS to report a non-zero speed when the device is stationary; this is due to small variations in the positional fix over time.

[x19]

Oh no

It's been around for years and accuracy seems to be down to a few centimetres.

Aircraft could fly without pilots if it didn't freak out the passengers.

And nothing ever went wrong!

[Ryeman]

And nothing ever went wrong!

The certification maintenance standards of all the technology makes it uneconomic in any case.

It's a theoretical standard more applicable to space reentry than cheap passenger miles.

If, for motoring purposes, it's not accurate, then the rock solid figure I get confirming the speedo's variation is otherwise difficult to rationalise.

I accept it, absent a better explanation.

Edited 28 November, 2016 by Ryeman