Speedometer calibration

[Butt0ns]

In my work I have to deal with EU law on a regular basis and the area I deal with is one of the most complex and outdated. Hence I've always had an interest in this kind of thing and calibrate my speedo on a regular basis using motorway distance markings which are every 100m on Irish motorways. I usually calibrate over 20 km or more.



Note the European regulation requires the speedometer to underestimate the actual speed at all times.



At all times a vehicles speedometer must indicate a higher speed than that actually travelled.



From experience the speedometer on the Insignia can be up to 10km/hr (above) out depending on tyre wear, at 120kph. See the link below.



This is the requirement for the speedometer and does not take account of the accuracy of external measurement equipment used by the enforcement authorities, (laser/radar etc) At the very minimum one can travel at the speed (V2):0<V1-V2<V2/10+4kph where '<' is equal to or less than and V1 is indicated speed and V2 is actual speed without fear of prosecution.



Note the requirement regarding tyre pressures etc......get out of jail clause!



In Ireland the vast majority of speeding fines are issued to vehicles travelling over 10kph over the prescribed limit. Offficial stats prove this.



http://eur-lex.europ...75L0443&from=EN



Just for info, I have never got a speeding fine.

[Butt0ns]

To put common sense on the above previous post in a:??



80 kph zone vehicle speedometer can read up to 92 kph

100 kph zone vehicle speedometer can read up to 114 kph

120 kph zone vehicle speedometer can read up to 136 kph



So if you are issued a speeding ticket below the above limits I would question the validity of the ticket.



I am not a person charged with enforcement of motor traffic speed limits nor do I encourage speeding but vehicle speedometers are legally required to under estimate speed so note the legal tolerances for speedometers fitted to motor vehicles. The above is an interpretation of the requirements and should not be seen as definitive of same.

[DeaniFLHT]

m8internet, on 30 January 2016 - 07:39 AM, said:
This is already covered by the Vehicle Construction Act here in the UK

An uncalibrated speedometer (like fitted to a car) is permitted a tolerance of 10%



Equally, speed detection devices also have a tolerance

For any speed offence the compound total has to be used

Hence why most crown offices will only pursue cases which are deemed to be "excessive speeding" and/or two or more offences occured

All of which makes the statements made by some authorities a few months ago, that they intended to enforce some limits without any tolerances, completely farcical.

[Butt0ns]

Sorry 'm8intrenet', I am not picking a fight but under EU requirements I do not believe there is a 'blanket' 10% tolerance on speedometers, the requirements are clearly set out in accordance with the EU directive which is enacted by each member state by national law. UK laws perhaps must be stricter than EU requirements. I may be wrong and open to correction. Please do so. My specialty is 2000/29/EC. Plant Health Directive. Oh what fun! Good to chat.

[Unilateral]

Generally, the UK doesn't like to have every single thing we do being controlled by the EU. We like to be able to make our own choices so maybe that's why we have different rules of tolerance/callibration?



Don't mean that in any offensive way, just observational from how people feel about being controlled by or even in the EU.

[Butt0ns]

I'm cool with all that. Dont like my Da telling me what to do never mind a suit in Brussels!

[Butt0ns]

Agree 100% with the above. Excellent work pfranks. With my 20" wheels on 245/35 tyres the difference between a new set of tyres (8mm thread) and those with 2mm thread at 100kmph is 2kph, due to the rolling radius difference. Similar discrepancies will arise through different tyre pressures, lower pressure giving higher indicated speed and higher pressure giving lower indicated speeds.

All other things being equal:

Most accurate indicated speeds (relative to true speed) will be with with new tyres at eco or max??pressure

Least accurate indicated speeds (relative to true speed) will be with worn tyres at low pressure



Stats not my strong point!

Hope my calcs for the above are correct or you'll be putting me right!

[Butt0ns]

pfranks, you look like a night owl like me - though I work days!

From what I know, GPS is generally super accurate though the signal is purposefully distorted by US. Inaccuracy is offset by differential signal from base points on land (as long as you have it switched on, on your GPS/sat nav). EU GPS (not live as yet) is coming, or is soon, on stream. From calibration of my speedo by distance over time, GPS is as accurate. There is no more an accurate measurement of a cars speed at a particular moment other than to measure distance (not odometer), over time at a constant speed, as the police do for evidential readings. My original point was for everyone to calibrate speedo to match your tyre size/pressure set up. You'd be surprised how inaccurate the speedo can be. And if the EU requirement for speedometer accuracy is observed how can one be prosecuted for exceeding the limit beyond that which is prescribed by EU law. Taking into count your statistics, (and I accept your stats 100%) if the law prescribes a limit then that is the limit and no less. That's the point I was making. If a speedo has to be accurate to e.g. <136 at a true speed of 120 how can a prosecution be upheld in a court of law if the prosecution charges at a speed <136.

Thanks for your time and input. I'm learning things.

What speed anyone chooses to travel at is up to them.

[Butt0ns]

If you trust the US military who control the signal. OK.

At that time the intention was relating to "Intentional degradation", the signal is still degraded. Randomly. Accuracy depends on time. One second it could be as accurate as possible and the next who knows. US military do not use same devices as you and??and can overcome this discrepancy.

At work We use GIS/GPS for our mapping systems. GPS/Differential GPS readings alone are not accepted.

Good to have open discussion, just want to see what other info others have on these things.

Fact remains your speedometer may read 136 kph whilst travelling at an actual speed of 120 kph. Whilst you know you are travelling at a speed greater than 120 kph what court will stand over such prosecution, taking into consideration factors such as; tyre pressure, atmospheric pressure, calibration of measurement equipment used by enforcement authorities etc. It is possible you may be travelling at 121kph and this would be well within calibration requirement of enforcement authorities.

Again I respect and approve all the points you have previously posted, I'm playing devils advocate here.

I'm learning as we go.

Thanks 'pfranks'