Practically Classics Forum

i borrowed a modern tomtom a few months ago & your speed is displayed on the screen... it kept bleeping & showing red until i worked out i was slightly breaking the limit...

I posted the entire section from the manual including a link to it and there is NOTHING in there about accuracy of speedometers. The EU has directives on the calibration equipment but not the speedometers themselves.

I will admit I frequently get things wrong but in this instance until you (or someone else) can back up their claims i'll stick with ACPO.

I posted a direct quote from the Construction & Use regs, to me, it couldn't be clearer on the matter of speedometer accuracy which - as OCO seems not to have noticed - states that the inaccuracy must be over the true speed but never below it, which is why an inaccurate instrument that reads below is no defence and can lead to a prosecution for that fault. Indeed it would have done had I contested it.

In case the quote doesn't count, I'll add a link in the hope that this will suffice, click the text to view the page:

Lord Whitty (Parliamentary Under-Secretary, Department of the Environment, Transport and the Regions; Labour) wrote:

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.

Please tell me how it is that there's room for debate in that? (WITHOUT citing Gavin Hetherington's post please, as he seems to be another who's unable to understand the words....)

Try this UN document, part 5.3 specifies accuracy etc:

http://www.unece.org/fileadmin/DAM/tran ... 039r1e.pdf

However, with the greatest respect, isn't it a lot simpler just to keep under the speed limit in the first place???

Quotes count for nothing without reference to their source. Everybody can use quote tags.

This is the actual act rather than a politician "quoting":
http://www.legislation.gov.uk/uksi/1986 ... tents/made
and NOWHERE IN IT IS THAT PASSAGE PRESENT!
A single line at the bottom of a revision to the EC directive states this:
The speed indicated must never be less than the true speed. At the speeds specified for the test in 4.3.5 above and between these speeds, there shall be the following relationship between the speed indicated on the dial of the speedometer
But then fails to give any further details

Tractorman's link does say +/- 10% +4kph and must not over read which is fair enough.

There is no reference ANYWHERE to that only being applied at 30mph

The police should still adhere to their own guidelines. Regardless of the manufacturers requirements (which only apply to vehicles made after 1984) there is no testing of speedometer accuracy so I find it hard to believe that anybody’s car has a gauge that is accurate.
A quick run up that road (trip to Tesco) for a speedo - tomtom test and my RR under reads at 30 with tomtom saying 30 and my speedo saying 32 and at 70 on the tomtom I am actually doing 73. OMG I MUST TURN MYSELF IN IMEDIATELY! (Or possibly check my tyre pressures, pull my speedo needle off and move it round a bit, etc.)

Oh dear, we are getting confused. Tractorman's quote says that the permitted over-reading must be less than or equal to 10% of the true speed plus 4km/h and it must not under read. Terry, your speedo is over-reading and is within the permitted tolerance, so you can relax

tractorman wrote: isn't it a lot simpler just to keep under the speed limit in the first place???

id be late for work

Maths was never my strong point, it does say "The speed indicated shall not be less than the true speed of the vehicle." but is followed by an equation that contradicts it.
Law is always open to interpretation which is why solicitors have a job.

Sticking to speed limits is always a good idea, however if you are doing 70 in to a head wind and the wind direction changes, you'll find yourself doing 72 before you can react. 0 tollerance with a policeman following you and you'd get 3 points.

TerryG wrote:Sticking to speed limits is always a good idea, however if you are doing 70 in to a head wind and the wind direction changes, you'll find yourself doing 72 before you can react. 0 tollerance with a policeman following you and you'd get 3 points.

And if you're holding your speed that close to the limit when there's a copper behind you then you're asking for trouble anyway, but as he'd have just been hit by the same gust of wind I would expect him to use his best judgement & observe your reaction to the changing conditions. If anyone here has genuinely been nicked for doing 1mph over please speak now, otherwise I think we need to stop running these hypothetical zero-tolerance situations; they're all based around a disturbing lack of roadsense, and as such are terrible strawmen.

We've got all the official rules & regs, including the ones that show the speed camera leeway calibrations are based around 10%+2mph; cameras don't have the same personal judgement as an officer, so are designed with tolerance in mind. While Construction & Use sets limits on variance, it's not easy to discover what your car's tolerances are; this has even been raised in Parliament: http://www.publications.parliament.uk/p ... 312w01.htm

Speedometer Accuracy
Lord Allen of Abbeydale asked Her Majesty's Government:

Whether, in the light of the increasing importance of speed limits, they have any plans to make it easier for the private motorist to have his speedometer tested for accuracy.[HL839]

Lord Whitty: 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.

Using 10% as a rule of thumb has been supported throughout this discussion. At no point has anyone said you should consistantly exceed the limit by that amount, but an older machine with some wear is more likely to allow your speedo to drift over the indicated limit without your car physically travelling at that speed, and has been compensated for by the caemra operators.

What a lot of bollocks we're going through for what was originallly just a joke URL...

Bollocks or otherwise, reading the previous pages would have served you well as that ways; you'd have known that I mentioned having been convicted for 31.7mph in a 30 limit, this was a result of a camera van's evidence and I believe that a human being would have used discretion. Nevertheless, L&B has a zero tolerance approach, as does the Northumbria force in its parts of Northumberland, whither in cases captured by those mobile cameras or ones involving human beings. The photos are available on the genuine speedsafe database, enter PKE514L, date 14/10/2010 to see the image, complete with 31.7mph shewn in the speed field.
Feel free to interpret the finger (front photograph) in any way you see fit!