Speedodometer accuracy - myth or fact?!!

Guy:Incognito

R_H_C_P wrote: »

ya, honestly if I were to see one more argument about foglights I think I will just give up on life or something :pac:

Should we not be trying to educate bad drivers rather than ignore them?

L-M

I just remembered something actually. A friend of mine restored a Mini, and long story short when she was up and running he brought it out onto the dual carrage way, and made his wife follow him to check his speedo. He was doing 60 mph, and she was doing 80mph to keep up:P

chopley

unkel wrote: »

Speedos legally can show any speed between the actual speed and the actual speed + 10% + 4km/h. So the speedo of a car doing an actual 160km/h can show anything between 160km/h and 180kmh (160+16+4).

Almost correct! For cars, the difference between the indicated speed and the actual speed must be less than or equal to 10% of the actual speed + 6 km/h. For motorbikes the tolerance is even wider: 10% of actual speed + 8 km/h.

The official formula used in the international agreements* is:

0 ≤ (V1 – V2) ≤ 0.1 V2 + 6 km/h (for cars)

So if you were actually doing 160 km/h, your speedo reading could be as high as 182 km/h (10% of 160 + 6)!!

See: Uniform Provisions concerning the Approval of Vehicles with regard to the Speedometer Equipment including its Installation

unkel

chopley wrote: »

Almost correct! For cars, the difference between the indicated speed and the actual speed must be less than or equal to 10% of the actual speed + 6 km/h.

Eh, no. It is 4km/h as I stated. Here is the legislation - see 2.3.7

unkel

Maybe this deserves a place in the main sticky? Threads about speedo accuracy seem to come up all the time now and everytime I have to search for the bloody legislation again

-Chris-

unkel wrote: »

Maybe this deserves a place in the main sticky? Threads about speedo accuracy seem to come up all the time now and everytime I have to search for the bloody legislation again

Anything to make your life easier Unkel!

chopley

unkel wrote: »

Eh, no. It is 4km/h as I stated. Here is the legislation - see 2.3.7

Look at the title of what you're quoting: "Directive 2000/7/EC of the European Parliament and of the Council of 20 March 2000 on speedometers for two- or three-wheel motor vehicles" Now how many wheels has your car got????

-Chris-

OK, if this is going to stay in the Charter, somebody'd better have a conclusive answer to this...

unkel

chopley wrote: »

Look at the title of what you're quoting

Look at the source of what you're quoting

EU regulations are stricter than the UN regulations you are quoting.

The EU legislation for the accuracy of the speedo for 2 and 3 wheel vehicles is the same as for all motor vehicles

Council Directive 75/443/EEC of 26 June 1975 on the approximation of the laws of the Member States relating to the reverse and speedometer equipment of motor vehicles

Now click on any of the html links and go to 4.4. You'll see it is 4km/h. This Directive was amended by Commission Directive 97/39/EC of 24 June 1997 adapting to technical progress Council Directive 75/443/EEC of 26 June 1975 relating to the reverse and speedometer equipment of motor vehicles (Text with EEA relevance) but 4.4 was not affected.

Note: don't select English as your language, cause the page is incomplete. Any other language will do

E.g. Italian:

4.4 . la velocità indicata non deve mai essere inferiore alla velocità reale . Alle velocità specificate nel punto 4.3.5 e a quelle intermedie , tra la velocità V1 indicata e la velocità reale V2 deve sussistere la seguente relazione :

O * V1 - V2 * V2/10 + 4 km/h .

chopley

unkel wrote: »

Look at the source of what you're quoting

EU regulations are stricter than the UN regulations you are quoting.

The EU legislation for the accuracy of the speedo for 2 and 3 wheel vehicles is the same as for all motor vehicles

Council Directive 75/443/EEC of 26 June 1975 on the approximation of the laws of the Member States relating to the reverse and speedometer equipment of motor vehicles

Now click on any of the html links and go to 4.4. You'll see it is 4km/h. This Directive was amended by Commission Directive 97/39/EC of 24 June 1997 adapting to technical progress Council Directive 75/443/EEC of 26 June 1975 relating to the reverse and speedometer equipment of motor vehicles (Text with EEA relevance) but 4.4 was not affected.

Note: don't select English as your language, cause the page is incomplete. Any other language will do

E.g. Italian:

Now it all makes sense. The legislation you cited earlier brings the requirements for 2 and 3 wheel vehicles into line with those for four wheel vehicles. I wasn't aware of this much earlier legislation, hence I assumed that the laxer international rules still applied to cars.

At least we've got the matter cleared up now And with references to boot

DanWall

Have I got this right? at a 100km/h my speedo could be 114km/h, so I am well covered for speeding over a little bit

unkel

It may have taken you 2 years, Dan. But you did get it right!

Iwannahurl

Fishtits wrote: »

Satnav or other GPS device will only record accurate speed when the receiver is traveling in a parallel plane to the satellite.

In common language, they're not accurate. Myth busted.

BTW, a good site to get an understanding on how GPS works is trimble.com, follow the tutorial.

I don't use satnav in the car so I have done no homework on the accuracy of these systems in the context of driving speed.

However, I thought the accuracy of GPS was due to the use of multiple satellites for determining position. In which case I'm not sure how a receiver could be travelling in a "parallel plane" to just one satellite.

Here's a webpage on tomtomforums.com claiming an accuracy of "0.1 mph away from deadly accurate".

DanWall wrote: »

Have I got this right? at a 100km/h my speedo could be 114km/h, so I am well covered for speeding over a little bit

AFAIK, the key elements of the EU directive on speedometers are:

• The indicated speed must never be less than the actual speed
• The indicated speed must not be more than 10 percent over the true speed plus 4 km/h.

These are tolerances, not absolute values. Therefore your speedometer reading might or might not coincide with the maximum variation allowed. In which case, you may not be "well covered".

Esel

DanWall wrote: »

Have I got this right? at a 100km/h my speedo could be 114km/h, so I am well covered for speeding over a little bit

Provided your wheel/tyre combination is the same diameter as the original equipment.... and shouldn't that 114km/h above be 110km/h?

unkel

Esel wrote: »

shouldn't that 114km/h above be 110km/h?

RTFT, Esel :pac:

DanWall

I contacted Garmen who said that the reason it reads faster than the speedo is for that very reason, it is much more accurate, its good to make a comparison, with speedo it can also vary a few percent depending on tyre wear

Matt Simis

DanWall wrote: »

I contacted Garmen who said that the reason it reads faster than the speedo is for that very reason, it is much more accurate, its good to make a comparison, with speedo it can also vary a few percent depending on tyre wear

Also note that unless you have 4 Sats connected on your GPS you are only in 2D Tracking and therefore elevation (altitude) is not taken into account, which skews the speed reading.

corktina

Matt Simis wrote: »

Also note that unless you have 4 Sats connected on your GPS you are only in 2D Tracking and therefore elevation (altitude) is not taken into account, which skews the speed reading.

whereas that is undoubtedly true, basic triganometry wil tell you that the "long " sides of the triangle stretch all the way up to a satellite and the "short" side is very very short indeed, making the inaccuracy due to (what I assume you mean) inclination incredibly small.

langdang

corktina wrote: »

whereas that is undoubtedly true, basic triganometry wil tell you that the "long " sides of the triangle stretch all the way up to a satellite and the "short" side is very very short indeed, making the inaccuracy due to (what I assume you mean) inclination incredibly small.

Haha, I'm picturing peoples heads exploding trying to understand this if maths/physics is not their thing:D

stevenmu

DanWall wrote: »

Have I got this right? at a 100km/h my speedo could be 114km/h, so I am well covered for speeding over a little bit

It's not safe to just assume that 114kph indicated is 100kph real speed, that's just the maximum it's allowed to overread by. Your speedo could be anywhere from 100kph to 114kph at a real speed of 100kph.

It will vary from car. My current car reads 108kph and my previous car read 112kph, both compared against a real speed of 100kph (by GPS).

stevenmu

langdang wrote: »

Haha, I'm picturing peoples heads exploding trying to understand this if maths/physics is not their thing:D

I'm actually trying to work out how this would affect speed cameras and garda hair-dyers.

I'm thinking that if you were driving down a 45 degree slope at say 120kph, with a camera/garda in the distance positioned such that the angle between your movement and the plane of the camera/garda gun is also 45 degrees, they would read your speed as only 60kph.

But I'm really not sure about that, so if anyone gets busted doing 240kph downhill on a motorway don't blame me

Matt Simis

stevenmu wrote: »

I'm actually trying to work out how this would affect speed cameras and garda hair-dyers.

I'm thinking that if you were driving down a 45 degree slope at say 120kph, with a camera/garda in the distance positioned such that the angle between your movement and the plane of the camera/garda gun is also 45 degrees, they would read your speed as only 60kph.

Ive also theorised something similar, if you are being tagged and monitored for speed as you approach a speed camera/gun, slowing down while changing lane would also reduce the perceived speed as some of that speed will be used laterally for the lane change?

langdang

Which way to the vectors forum?

stevenmu

Makes sense. AFAIK the radar devices use doppler shift to measure speed, which effectively meaures how quickly you are moving towards or away from the radar, not how quick you move relative to the ground. Similarly the laser guns work by measuring how quickly your distance to them changes.

I think that you could drive in a circle around either device as quickly as you like and it would read a speed of 0.

unkel

corktina wrote: »

basic triganometry wil tell you that the "long " sides of the triangle stretch all the way up to a satellite and the "short" side is very very short indeed, making the inaccuracy due to (what I assume you mean) inclination incredibly small.

Or another way of saying that is that most inclines/declines are no more than a few percent

Mr Benevolent

stevenmu wrote: »

I think that you could drive in a circle around either device as quickly as you like and it would read a speed of 0.

Yup, effective change in distance would be zero, hence zero kph. Interesting.

Guy:Incognito

Matt Simis wrote: »

, slowing down while changing lane would also reduce the perceived speed as some of that speed will be used laterally for the lane change?

Super, now we're going to have idiots on motorways weaving in to other lanes as well as jamming on when they see the lines on the road, even when they were already 10kmph under the limit to begin with.

Tea 1000

Matt Simis wrote: »

Ive also theorised something similar, if you are being tagged and monitored for speed as you approach a speed camera/gun, slowing down while changing lane would also reduce the perceived speed as some of that speed will be used laterally for the lane change?

In theory that's true, but in practice the radar guns would be too quick registering your speed, and the laser guns have got it long before you've seen the cop, thought about reacting, moved the steering wheel, lifted your foot off the accellerator... etc!

Iwannahurl

Sounds like people may be referring to the Cosine Effect. It always works in favour of the motorist, but I presume speed camera operators are trained to allow for it.

Still, always worth thinking about new ways to beat the system.

Voodoomelon

I compared my speedometer speed compared to my Roadhawk GPS speed over 5 miles and my E38 overreads by exactly 5%.