speedometer inaccuracies

[richardg8jvm]

Hi All

Thought today I'd check against GPS the speedo and maxidot display

As I noticed others getting very impatient with me doing a displayed 50mph in the road works on the M5 near Worcester, UK

 

I found that 50mph from the GPS was 55 ish displayed on the speedo, ie a 10% error, maxidot was a little closer.

Which had me wondering as my last car was only very close when the tyre had a full new tread.

 

The 225/50R17 tyres on my Yeti adventure equate to the US  size of 25.8x8.5 R17, diameter/width radius

25.8" = 657.86mm  

So calculating the circumference, converting that to yards and dividing 1760 by the answer gives the amount of revolutions per mile.

Factoring in 1mm of wear to the calculations gives a figure of 0.234% error/mm tread wear

My tread depth on the front tyres is 6mm

 

So if you put that error in at 50mph it doesn't even give a 1mph error.

 

As I suspect the speedometer is a moving coil meter 5% linearity would be its best, and as its mass produced probably worse

I'm surprised that Skoda has not made corrections to this.

The maxidot speed indication is slightly more accurate but still reads low. this would indicate the final drive sensor provides a digital output,

which could be easily corrected in the digital to analogue conversion process to correct the speedometer linearity at several points.

Maybe the teutonic big brother is trying to make us drive slower 

 

I'd be interested how much others find the Speedometer error is ?????

with the amount of average speed cameras around it pays to know exactly what you are doing.

As well as those idiots using mobile cameras who have no idea of cosine correction when using on bridges overs roads or when hiding in the bushes at the side of the road.

 

 

 

[Llanigraham]

What's the problem?
The speedo overreads and is within the legal paramenters. If you are doing an indicated 50 in a 50 limit you will not be breaking the Law.

 

And there is a simple answer to your last slightly offensive last sentence; don't break the Law!

[langers2k]

The speedometer needle is a stepper motor, the inaccuracy is by design.

 

European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. If you are travelling at a true 50mph, it can display anything between 50mph and ~57.5mph

 

Although you'd expect all 225/50 R17 tyres to be the same size, they really aren't which can also add errors

[33q]

Hi All

Thought today I'd check against GPS the speedo and maxidot display

As I noticed others getting very impatient with me doing a displayed 50mph in the road works on the M5 near Worcester, UK

 

I found that 50mph from the GPS was 55 ish displayed on the speedo, ie a 10% error, maxidot was a little closer.

Which had me wondering as my last car was only very close when the tyre had a full new tread.

 

The 225/50R17 tyres on my Yeti adventure equate to the US  size of 25.8x8.5 R17, diameter/width radius

25.8" = 657.86mm  

So calculating the circumference, converting that to yards and dividing 1760 by the answer gives the amount of revolutions per mile.

Factoring in 1mm of wear to the calculations gives a figure of 0.234% error/mm tread wear

My tread depth on the front tyres is 6mm

 

So if you put that error in at 50mph it doesn't even give a 1mph error.

 

As I suspect the speedometer is a moving coil meter 5% linearity would be its best, and as its mass produced probably worse

I'm surprised that Skoda has not made corrections to this.

The maxidot speed indication is slightly more accurate but still reads low. this would indicate the final drive sensor provides a digital output,

which could be easily corrected in the digital to analogue conversion process to correct the speedometer linearity at several points.

Maybe the teutonic big brother is trying to make us drive slower 

 

I'd be interested how much others find the Speedometer error is ?????

with the amount of average speed cameras around it pays to know exactly what you are doing.

As well as those idiots using mobile cameras who have no idea of cosine correction when using on bridges overs roads or when hiding in the bushes at the side of the road.

 

I happen to think that 'these idiots in the bushes' etc are a good thing.

 

On the Cosine correction bit.....I cannot beleive this challenge has not been well and truly been tried by many law breakers and quite rightly been an win to the Police.

[Frenchtone]

You've got your answer then, drive at an indicated 55mph to be at 50, good luck with that, I did the same here a la frog, autoroute camera flashed me in a 90kmph, docs showed my speed just over at 91 so I must have passed at an indicated 100 - 102, that's life, cost me 40€, but no points, still have my UK licence!!

[richardg8jvm]

I see someone thinks cosine correction is an excuse and not a fact

 

It certainly is a FACT.

If Your speed is measured over a distance the measurement device must be in the same axis.

moving the measurement device away from that axis moves it at right angles from that axis and  

Pythagoras's Law applies.

Something accepted for the last 2000 years +

A problem is now that many camera partnerships believe that using infra red lazers eliminate this and it only applied to microwave measurement devices.

However, this is wrong as the velocity of light and radio waves is the same.

The standard of scientific education in the UK is appalling now.

[Llanigraham]

I see someone thinks cosine correction is an excuse and not a fact

 

It certainly is a FACT.

If Your speed is measured over a distance the measurement device must be in the same axis.

moving the measurement device away from that axis moves it at right angles from that axis and  

Pythagoras's Law applies.

Something accepted for the last 2000 years +

A problem is now that many camera partnerships believe that using infra red lazers eliminate this and it only applied to microwave measurement devices.

However, this is wrong as the velocity of light and radio waves is the same.

The standard of scientific education in the UK is appalling now.

 

 

Oh do love it when someone says FACT in a post!!

And I have a simple answer; PROVE IT?

How do you know that this hasn't been written into the calculations the device makes? Unless you have access to the programming of the devices you cannot say that.

[nomdeplume]

I see someone thinks cosine correction is an excuse and not a fact

 

It certainly is a FACT.

If Your speed is measured over a distance the measurement device must be in the same axis.

moving the measurement device away from that axis moves it at right angles from that axis and  

Pythagoras's Law applies.

Something accepted for the last 2000 years +

A problem is now that many camera partnerships believe that using infra red lazers eliminate this and it only applied to microwave measurement devices.

However, this is wrong as the velocity of light and radio waves is the same.

The standard of scientific education in the UK is appalling now.

The UK police will generally allow a tolerance of actual speed +10% +3mph. This more than compensates for your cosine correction hangup.

[longedge]

Once you've taken relatives to ID a body in a mortuary viewing room, there because someone was driving too fast, you tend to see things in a different way to most other people. Seeing the devastating aftermath of a death caused by excess speed is very sobering. The sad thing is that speed limit enforcement nowadays seems in some cases to be an income generator rather than the life saving, road safety activity that it should be.

[Ryeman]

Pardon my higorance, but how can a rate of closure be over represented by being offset from a straight line?.

[Ryeman]

Once you've taken relatives to ID a body in a mortuary viewing room, there because someone was driving too fast, you tend to see things in a different way to most other people. Seeing the devastating aftermath of a death caused by excess speed is very sobering. The sad thing is that speed limit enforcement nowadays seems in some cases to be an income generator rather than the life saving, road safety activity that it should be.

Those who want to get to the 'scene' first want cameras depicted as revenue raisers which they only do if you consider yourself special.

They have lowered the road toll over many years (in adjusted terms) and are therefore improving safety for those who don't see ourselves as special.

The interweb is a dangerous device also

[langers2k]

I see someone thinks cosine correction is an excuse and not a fact

 

Not sure what cosine correction has got to do with the speedometer in your car.

 

It can only make your speed appear lower to someone measuring it, the larger the angle, the lower your speed will appear to them.

 

If you want to make sure you're staying on the right side of the speed limit, just use the speedometer. If you want to do exactly the speed limit, invest in a separate calibrated speedometer. Even a simple well calibrated bicycle speedometer should give a good speed indication when travelling in a straight line...

[richardg8jvm]

Pardon my higorance, but how can a rate of closure be over represented by being offset from a straight line?.

Look up Pythagoras Law, and very basic trigonometry.

 

 

The Police allow for the inaccuracies in car instrumentation and make allowances above that.

 

BUT

the camera partnerships have civilians operating equipment who are not properly trained in how to use the equipment.

I haven't advocated driving in excess of the speed limit, just know what speed you are driving at.

 

As for proving FACTS, there is no need to proove trigonometric laws which have existed for centuries and are accepted by all institutions.

It doesn't matter what algorithms are used to calculate speed , if the equipment is not used in accordance with guidelines it will not give accurate results.

[Ryeman]

"Over represented "?

The error is always in the driver's favour.

Does the operator apply an offsetting factor?

[Llanigraham]

Look up Pythagoras Law, and very basic trigonometry.

 

 

The Police allow for the inaccuracies in car instrumentation and make allowances above that. (1)

 

BUT

the camera partnerships have civilians operating equipment who are not properly trained in how to use the equipment. (2)

I haven't advocated driving in excess of the speed limit, just know what speed you are driving at. (3)

 

As for proving FACTS, there is no need to proove trigonometric laws which have existed for centuries and are accepted by all institutions.(4)

It doesn't matter what algorithms are used to calculate speed , if the equipment is not used in accordance with guidelines it will not give accurate results. (5)

 

Oh dear you have got a bee in your bonnet, haven't you. Been booked for speeding lately, perhaps?

 

1/ Well that is one thing we can agree on. ACPO guidelines are currently limit +10% + 3mph, so 30 is 36 actual. Using your calculations your speedo would be showing about 40 at that speed. Perhaps if you drove at 30 indicated it wouldn't be a problem!

 

2/ Really? And you have evidence of that assertion? And what about all the cameras that are used by Policemen? I think the polite term is utter rubbish! (And yes I have spent time in a Police manned camera van!)

 

3/ Drive to the speed indicated on the car's instrument then, not a problem is it?

 

4/ So you cannot prove that the instruments do not have that built into their calculations, but are willing to still make the same statement. 

 

5/ See 2 above.

 

I suggest you volunteer to spend a weekend with your local Police or Ambulance service, attending RTC's, to see what the effect of speed is. It is not pleasant seeing dead and mangled bodies.

[aerofurb]

All three of my Yetis have had speedometers that over read by 5% when measured at various speeds against GPS.

Various stages of tyre wear - this can only have a negligible effect.

I know the speed limits and drive accordingly.

Edited 23 November, 2016 by aerofurb

[Ryeman]

To get booked in Oz you basically need to be indicating 10kph over to get an 'alleged' speeding charge of approximately +3 (allowing for the -2 correction for radar error).

[richardg8jvm]

No the error is not in favour of the driver, just the opposite.

 

Calculate it yourself and you will see, do not forget it is not one triangle you calculate the hypotenuse from, 

but two , one at the start of measurement and one and the end of measurement, the difference in the patterns will leave a obtuse

triangle, with the shortest side being the distance travelled. depending on the pulse repetition rate of the measuring device will result in the measured 

distance. Plenty of Radar theory papers available on line, many are mathematically intense.

The maths becomes a lot easier to understand if you take the range of acquisition as units of time instead of distance, ie divide the distance of the hypotenuse in metres

by the velocity of light in free space , 300,000,000 mtrs/sec

 

Aerofurb you are correct about tyre wear, my calculations on a 17" rim, gave an error of 0.234%/mm wear

 

Graham ..read what was posted, and no I have not had a recent speeding ticket  !

[Ryeman]

Applying the KISS principle

The 'gun' UNDER READS unless it's going to be flattened by the detected vehicle.

Cheaper to pay up perhaps.

[Ryeman]

If I fly to a DME sight via a curve flight path I will get there sooner than a straight one apparently and my GS will be higher also?

Sounds like a way of getting to Mars much quicker.

[Llanigraham]

I'm beginning to wonder whether you know how the radar and laser speed cameras work!

 

The answer to all of this is quite simple:

 

DO NOT BREAK THE SPEED LIMIT!!

[Ryeman]

Hmmm

Sounds like trigonometry without the clock factor.

It's not about engineering drawings and the measurement of the hypotenuse.

It's clock based 'rate of closure'

[muddyjim]

No the error is not in favour of the driver, just the opposite.

 

Calculate it yourself and you will see, 

I don't need to; this web page explains all the maths and there is even a online calculator that shows that the radar measured speed is always LOWER than the true speed (in other words in the drivers favour). As an example, if the angle that the gun is pointing at the car in relation to the cars direction of travel is 30%, a car travelling at a true 70 mph will only show a reading of 60.6mph on the radar...

 

http://copradar.com/preview/chapt2/ch2d1.html

 

 

Jim

Edited 23 November, 2016 by muddyjim

[longedge]

the camera partnerships have civilians operating equipment who are not properly trained in how to use the equipment.

 

Admittedly it's getting on 20 yrs since I retired and I'm out of touch. I didn't realise that civilian staff were now operating speed checks for prosecution purposes in some areas. Whereabouts is that exactly? How were you able to discover that these civilians are not properly trained?

 

I can only say that before I retired and I was working within our Traffic Division, the training was rigorous and a final test had to be succesfully completed before we were allowed to use the then current speed checking equipment. The guidelines to which we worked were the same as the current ACPO recommendations as mentioned by Llanigraham.

 

There were many times when I was called worse things than an idiot . Sticks and stones etc. etc. - I've seen the bodies on the slab!

[PirateSyrett]

Many apologies for going back to the original question, but..... our Yeti's speedo hardly over reads at all! In fact it's astonishingly close to steady state GPS measurements on relatively new tyres at the right pressure.

I'd be most wary about automatically assuming any additional margin to exceed speed limits because in our car it's just not there!