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 Post subject: Speedo Accuracy
PostPosted: Mon Oct 20, 2008 17:33 
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Looking at posts on "Help, I'm being prosecuted " ---strikes me that me that as a user group - me got a rough deal -

After all - the very thing we rely on (according to HMG stats ) to "keep us safe " ----our SPEEDOMETER .
How accurate is it - Perhaps we should be asking for some system where we could have it's accuracy checked .(without it being made compulsary ,of course )

After all - provision is made for the Police to check their's .

So what's good enough for the enforcement ----fairs fair - should be good enough for Joe public .


Perhaps a campaign to shout about Speedo calibration might (in Navy terms ) throw a smokescreen on the bows of HMG .

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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Oct 21, 2008 14:15 
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I suppose the official answer might lie something along the lines that as speedos can only legally over read you are safe because you will always be going slower than the limit. Assuming you drive to the limit as displayed on your speedo of course.

As for calibration, I used to get a passenger to time across the 100m posts on a motorway while I held a steady speed, now I use a sat nav. Even with a sat nav the reading errors from it and the speedo are going to leave you with an uncertainty of around +/- 2mph at best which enough to get you nipped.

In my main car the analogue speedo is quite small and hard to read, it only has markings at 10mph intervals and the needle is relatively thick so I tend to use the digital readout, which is in Kph, for better reading accuracy.

Even if you have a calibration for your speedo unless you can get it corrected, not forgetting the ongoing change due to tyre wear, you will have to know or calculate the true reading for each speed limit.

So all I have to do it add 10% to the posted limit, leaving the +2 for safety, add another 7% to correct the speedo error then convert that into Kph and remember where the needle is pointing. This of course assumes the offset is low enough to ignore and that the speedo is linear.

To summarise, it will not be simple or particularly useful to most drivers to have their speedo calibrated, in fact except for trying to avoid prosecution while pushing the legal limits I doubt it is really necessary. Which, to my mind, helps reinforce the point that speed is not an accurate measure of safety. If we do have calibrated speedos will the acpo prosecution limits be lowered?

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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Oct 21, 2008 21:38 
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toltec - sorry ,mate not the answer I was looking for .

BIB are allowed to calibrate their speedos - so basically why can't we . a pilot can expect to have his altimeter checked , a boat captain can expect to have his depth equipment checked - WHY -so that their vessels are safe .So why cannot a defence of an uncalibrated speedo be used . So why is their no independant calibration standard for speedos.
We have national standards for equipment calibration - where all equipment has to be traced back to national standards . But then ,these people place the safety of a child in DANGER if a driver IS EXCEEDING A SPEED LIMIT HE CANNOT BE EXPECTED TO KNOW IS ACCURATE .So -either the system is wrong , or THE FACTS ABOUT SPEED BEING A FACTOR ARE WRONG .(sORRY - TAKING THE PI$$ ON THE SYSTEM USED :lol: )

Pilot hits peak he thought he was below ,because of faulty(poorly calibrated gauge) ---by by licence ( :bounce1: AWAY)
Ships Captain hits rock due to poorly maintained navigatin /depth finder- licence sunk .(GLUG GLUG).
Car driver expeeds speed limit due to faulty speedo - WHOOPS - DRIVER ERROR -WE CAN HAVE HIM DONE -although he was driving on instruments ,TRUSTING THEM TO BE TRUE .


See the INJUSTICE ???????

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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Oct 21, 2008 23:25 
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I don't want to argue at cross purposes botach, it is quite possible that I am being a bit pedantic about what calibration is but not explaining that well enough in my post. Do you mean motorists should be able to have their speedos adjusted to read correctly, i.e that the instrument itself should be capable of being adjusted and that the opportunity/facility to have this done is made available? I could see that as being useful where a calibration would not be to most drivers.

I do like the idea of trying to use the lack of accuracy of the instrument in an average car to highlight the absurdity of the using speed as a measure of safety. The downside of having accurate speed measurement could be a tightening of the acpo limits. In my usual way I am just looking at both sides of the idea.

Are the police speedos just calibrated or also adjusted to show a correct reading? Anybody able to clarify?

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 Post subject: Re: Speedo Accuracy
PostPosted: Thu Nov 20, 2008 00:07 
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Speedos in modern mass-produced "type" approved cars are allowed to over read by 10% plus 2.5MPH. As has been said, they are not allowed to UNDER rear AT ALL. There's an EC Directive about it, 75/443/EC. The ship's depth sounder or aircraft altimeter analogy would therefore be that ships could only have depth sounders that recorded the water as being a bit shallower than it really was and planes could only have altimeters that recorded the ground as being a bit nearer than it really was. Under our Construction and Use Regulations, it is an offence to drive a car with a defective speedo. If you try to claim you were speeding because your speedo was defective, you just add a second offence to your speeding offence!

Heavy goods vehicles can get their tachographs re-calibrated but there's no mechanism for doing so on a modern car speedo. Quite often, the error the speedo records is non-linear anyway, depending on the speed travelled at. Also, the speedo's accuracy will vary with tyre size, pressure and level of wear.


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 Post subject: Re: Speedo Accuracy
PostPosted: Thu Nov 20, 2008 10:59 
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Satnavs are the easiest to check on flat ground. Most people would have access to one that either they owned or a friend owned. There are also those numerical speed indicators popping up in quite a few places. Interestingly I notice some speedos seem to be calibrated at 30 to be spot then under read higher up and others seem to be spot on at 70 and under read all the way down.


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 Post subject: Re: Speedo Accuracy
PostPosted: Sun Nov 23, 2008 10:46 
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What annoys me is the roadside warning signs that flash up a speed warning when you are supposed to have exceeded the speed limit e.g. 50MPH. I don't know if it's just by me but whenever I am travelling at exactly the limit e.g. 50MPH they keep flashing me and others to slow down...and I drive two different vehicles with the same results...is it a government ploy to make us drive even slower than we need to to? I would be interested to hear from others if they have experienced this. The calibration on them is obviously TOO sensitive.

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.


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 Post subject: Re: Speedo Accuracy
PostPosted: Sun Nov 23, 2008 10:50 
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With speedo calibration, I read somewhere that speedos can read LOW at lower speeds, e.g. 30MPH so when you think you are doing 30MPH you could actually be doing 33MPH and yet read high at higher speeds, e.g. 74 MPH at a true 70MPH, is this just on older cars?

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.


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 Post subject: Re: Speedo Accuracy
PostPosted: Mon Nov 24, 2008 00:41 
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I'm pretty certain it was never true - EVER, for ANY cars! It's most certainly NOT true for modern cars (i.e. built in the last 20 years or so) at 30! I'd have to dig out a copy of the Directive to be sure but there MIGHT be some leeway at 5 or even 10 MPH (some speedos simply can't read that low - accurately or otherwise!) but certainly not true at 30.


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 Post subject: Re: Speedo Accuracy
PostPosted: Mon Nov 24, 2008 00:45 
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graball wrote:
What annoys me is the roadside warning signs that flash up a speed warning when you are supposed to have exceeded the speed limit e.g. 50MPH. I don't know if it's just by me but whenever I am travelling at exactly the limit e.g. 50MPH they keep flashing me and others to slow down...and I drive two different vehicles with the same results...is it a government ploy to make us drive even slower than we need to to? I would be interested to hear from others if they have experienced this. The calibration on them is obviously TOO sensitive.


Yes there's one near a school not far from me. The road is a single carriageway NSL road but the sign flashes the message "slow down!" at anything over about 40. Obviously, I haven't got a problem with it doing so in school hours and it wouldn't be sensible to drive past there at 40 at chucking-out or dropping-off time in any case, but what annoys the hell out of me is that you can be driving past at 2.00 in the morning and next thing this ultra-bright blaze of light flashes at you and you're driving blind for the next few yards!


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 Post subject: Re: Speedo Accuracy
PostPosted: Mon Nov 24, 2008 14:13 
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Mole wrote:
graball wrote:
What annoys me is the roadside warning signs that flash up a speed warning when you are supposed to have exceeded the speed limit e.g. 50MPH. I don't know if it's just by me but whenever I am travelling at exactly the limit e.g. 50MPH they keep flashing me and others to slow down...and I drive two different vehicles with the same results...is it a government ploy to make us drive even slower than we need to to? I would be interested to hear from others if they have experienced this. The calibration on them is obviously TOO sensitive.


Yes there's one near a school not far from me. The road is a single carriageway NSL road but the sign flashes the message "slow down!" at anything over about 40. Obviously, I haven't got a problem with it doing so in school hours and it wouldn't be sensible to drive past there at 40 at chucking-out or dropping-off time in any case, but what annoys the hell out of me is that you can be driving past at 2.00 in the morning and next thing this ultra-bright blaze of light flashes at you and you're driving blind for the next few yards!

i think you are taking that a bit far
are u seriously suggesting these things are so bright they can do this
i think u should have your eyes tested
sounds like you are complaining for the sake of it


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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Nov 25, 2008 00:44 
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Most of what I moan about on here is, of course, "complaining for the sake of it"! If I wanted anything DONE about it (or, more importantly, if I BELIEVED I could get anything done about it!) I'd take it up with the appropriate authorities. As it is, venting spleen on a forum is a form of R & R.

You'll be reassured to know that I have, in fact, as a wearer of glasses, had a fairly recent eye test and they seemed to be working OK.

As for my comments about the sign, I'm afraid I stand by them. I know I'm not the only one who feels that way as friends and neighbours have commented on the same signs too. They really are quite unpleasant - especially on a moonless night when your eyes are used to whatever level of brightness the headlights can manage. While it's pretty easy and relatively safe to drive a few yards "blind" (Obviously not TOTALLY blind but for the sake of pedantry, let's say "with significantly reduced night-vision"), the question is: Are these signs making things better, worse, or making no difference either way? My feeling is that by flashing all the time, day and night, they loose their usefulness. It's not an obvious place for a school - it's not like it's in a village, so the fact that there is something to warn drivers not familiar with the area is a good thing. Unfortunately, by flashing ourside the times of day (and year) when they would provide a useful and pertinent warning, I think they make things WORSE.


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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Nov 25, 2008 15:16 
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I was interested to see the comments about Sat Nav's.

My mother told me the other week that she was driver her and a friend and the friend had a Sat Nav which was saying that she was travelling at 77mph on the motorway when her speedo was reading 70mph.

She drives a Toyota Corolla that is just over one year old.

Any suggestions or thoughts on that?


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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Nov 25, 2008 23:59 
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lynladd wrote:
I was interested to see the comments about Sat Nav's.

My mother told me the other week that she was driver her and a friend and the friend had a Sat Nav which was saying that she was travelling at 77mph on the motorway when her speedo was reading 70mph.

She drives a Toyota Corolla that is just over one year old.

Any suggestions or thoughts on that?


Try a second sat-nav to confirm the reading?

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 Post subject: Re: Speedo Accuracy
PostPosted: Wed Nov 26, 2008 01:22 
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Assuming the car has OE wheels and tyres and the two sat-navs agree with each other, it looks like there's a problem with the car. It ought to be reported to the dealer and if she doesn't get satisfaction, it needs to be brought to the attention of toyota GB. I'd be very surprised if it WAS a mis-build but if it is, Toyota would have a "Conformity of Production non-compliance". If she was absolutely sure that it was under-reading, and she got no co-operation from the manufacturer, she could threaten to inform the Vehicle Certification Agency. They have the power to withdraw the vehicle's type approval (if it was issued in the UK), which means they wouldn't be able to sell that type of car - so they'd probably take that threat quite seriously! If it wasn't issued in the UK they could still make trouble for whichever EC Member State DID issue it!


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 Post subject: Re: Speedo Accuracy
PostPosted: Wed Nov 26, 2008 08:08 
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I think that she should calibrate the speedometer the old fashioned way - a passenger with a stop watch on a quite motorway - before escalating the problem to that level. GPS does have its own problems, especially refresh rate. If she was braking, even quite moderately, such a difference would be usual. One has to be travelling at a steady speed to get a meaningful GPS speed reading. That is the advantage of analogue instrumentation.

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 Post subject: Re: Speedo Accuracy
PostPosted: Tue Dec 02, 2008 13:55 
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Thanks for all the replies, I will pass them on to mother and leave her to decide what she wants to do.

Lyn


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 Post subject: Re: Speedo Accuracy
PostPosted: Wed Dec 24, 2008 13:37 
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By law car speedometers have to read plus or minus 10% at 30 Mph, most Plod take this into account when dishing out speeding fines ( Lancs 10% + 2 Mph ) which means that you could technically get away with 77 Mph on a motorway. If you wish to check the accuracy of your speedometer its quite simple, just follow a HGV on the level motorway at an accurately governed 56 Mph, ( plus or minus 1% ) my speedo reads about 60, most car speedo's read faster than the actual speed.


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 Post subject: Re: Speedo Accuracy
PostPosted: Wed Dec 24, 2008 14:30 
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Quote:
most car speedo's read faster than the actual speed.


My understanding of the law is that the speedo must not read less that the actual speed but my be 5% + 10kph fast



From Hansard
Lord Whitty (Parliamentary Under-Secretary, Department of the Environment, Transport and the Regions; Labour) | Hansard source

The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.

A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests.

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 Post subject: Re: Speedo Accuracy
PostPosted: Mon Oct 12, 2009 18:34 
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toltec wrote:
I don't want to argue at cross purposes botach, it is quite possible that I am being a bit pedantic about what calibration is but not explaining that well enough in my post. Do you mean motorists should be able to have their speedos adjusted to read correctly, i.e that the instrument itself should be capable of being adjusted and that the opportunity/facility to have this done is made available? I could see that as being useful where a calibration would not be to most drivers.

I do like the idea of trying to use the lack of accuracy of the instrument in an average car to highlight the absurdity of the using speed as a measure of safety. The downside of having accurate speed measurement could be a tightening of the acpo limits. In my usual way I am just looking at both sides of the idea.

Are the police speedos just calibrated or also adjusted to show a correct reading? Anybody able to clarify?



I believe that only the traffic police speedos are recalibrated & if i am correct it is done twice a year.


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