Speedometer Error

A bit of Sunday fun.

What's an acceptable error in a car speedometer?

There's a twist.

I seem to remember hearing the police turn a blind eye if you are over the limit by 10% + 2mph but I'm guessing that's not the answer you want

I always understood it to be +10% -0% i.e. the speedo can overstimate your speed, but not underestimate it.

However I think Euro regs get involved too;

See section 19 of **LINK**

19 Speedometers

1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at all speeds mph. up the maximum design speed of the vehicle.

2. For all true speeds up to the maximum design speed of the vehicle, the true speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the maximum design speed if lower), the difference between the indicated speed and the true speed shall not exceed—

V / 10 + 6.25mph where V=the true speed of the vehicle in mph.

This item shall not apply to vehicles having a maximum design speed of less than 2.5

There's also stuff related to mopeds etc.

A particular parliamentary answer clarifies(?) things further.

Speedometer Accuracy

AnchorLord 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]
AnchorLord 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.
AnchorA 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.

I bet you wish you hadn't asked

Many years ago i used to calibrate speedos, then you adjusted them using a magnetic field. Tolerances always stuck in my mind as 7% at 60mph, now this could be + or - and could be + or - from 0 to 60 as well.

Now of course we have digital speedos and I am sure they are run from a signal from the gearbox of a car so I think the only thing to affect the reading is tire size.

A an aside we tested RMP speedos and I always erred to let them read lower so if they came across a squaddie doing 33 they were doing 30.

Clive

It seems to be a lot better than it used to be. Ford were famed for there cars showing 100mph at below 80. People complained about being passed by a high speed coach link from B'ham to London prior to speed limits. They also seemed to calculate acceleration times in a vacuum.

The makers still do some odd things. I've spent many hours working a MIRA. It's interesting to see what they get up to. Usually understandable as some of the tests they have to do are in a sense impossible. To many variables.

I'd guess that all cars now show the same range of errors but tire pressure can make a difference and I would reckon around +10% possibly decreasing a little at higher speeds over the UK's limit. One of the magazines used to publish what the errors were in their tests and reviews. Some had rather low errors.

I have heard of people being booked for 32MPH in a 30 limit. Most people reckon this is because they didn't argue about it. I think it depends on what number the police decide to choose.

John

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Edited By John W1 on 16/08/2015 13:06:16

Well having been a persistent speed offender and attended 3 Speed Awareness courses, I was told the acceptable error was 10% irrespective of the speed....However for the purposes of prosecution or the issue of SAC the tolerance on speeds are 10% + 2 ....ergo if you are caught found doing 35 mph in a 30 limit, or not exceeding 42 mph, it was down to the Chief if he was feeling generous enough to allow you on an SAC..

Many moons ago I installed some machinery in a speedo manufacturers site in the midlands and I asked such a question and was told all speedometers are 10% fast so when your travelling at indicated 70 mph you are actually doing 63 mph, assuming SatNav's have a certain amount of accuracy all of the vehicles I drive indicate around 10%

According to my SatNav (when travelling in a straight line!) my Skoda Superb speedo reads about 5% high.

In a 50mph "average speed" zone undergoing motorway maintenance if I set the cruise control to 50 according to the SatNav then I tend to be going slightly faster than most of the rest of the law abiding citizens.

Rod

In the 50 zones on the motorway I always tuck in behind a HGV.

Reason is HVG's have to have the tacho calibrated every year by law so they have probably the most accurate measurement on vehicles on the road and because they do this for a living they are not going to jepodise their licence for the sake of a couple of mile per hour.

Result is as Rod say you go just this tad faster leading me to think that 'normal' vehicles are calibrated on the side of error.

Interesting responses! My old car used to pretty much agree with the GPS give or take a couple of mph, but my Ford now reads 4 or 5 mph high.

I thought that a group of folks who expect sub-thou accuracy in their machinery might want similar results from their car speedos!

Someone mentioned type size and one or two tyre pressure - clearly plenty of room for variation there, what I twiggged the other day is the difference in tyre diameter (and therefore speedometer reading) with tyre wear between 8mm new tyre and 1.6mm (legal limit), total 12.8mm or about 2%.

Clearly no speedo based on tyre rpm can be guaranteed better than a few %, but it's interesting to realise just how much variation tyres contribute.

.

Why ?

To go back to Neil's statement why would they want sub micron accuracy for an old clunker of a hobby lathe ?

It's because they do or they have read some bull$hit somewhere that tells them they need to. Some twonk somewhere convinces himself he can turn all day to 2 microns and because they don't know better they assume that that is the standard they have to achieve.

My local m/c speed cop told me last year- 10percent + 3mph. A friend of mine was stopped for speeding at 33mph, when he queried it with the patrol car driver he was told- I'm not doing you for speeding, I'm doing you for not seeing me behind you. He reckons he doing about 40, so deserved it.

John S,

Reason is HVG's have to have the tacho calibrated every year by law so they have probably the most accurate measurement on vehicles on the road and because they do this for a living they are not going to jepodise their licence for the sake of a couple of mile per hour.

They may be calibrated, but my experience is that a lot of HGV drivers do not stick to their nominated speed limits. I think some of this is down to the drivers knowing where the cameras are located, including the mobile ones, and hence charging on. Certainly I've been followed on the A66 at about 60mph when the HGV limit was 40mph (and yes, I know it's recently gone up to 50mph in England) and the A75 through Dumfries & Galloway is notorious for fast driving HGV's, especially the foreigners.

Regards,

Peter G. Shaw

Peter,

i was referring to the 50 mph average speed camera's not general speed limits.

The section I am referring to is on the M6 and you go through 3 sets of camera's and it's impossible to travel at greater than the listed speed without being clocked.

HGV are fitted with speed limiters set to 56mph and have been for a long time now. There was talk of allowing 60mph on dual carriageways that may have happened but as they are electronically limited not sure how many have been changed etc. Their speed limit on single carriageways should be 40mph.

I understand that there may be a 10 sec over taking time limit as well - totally disregarded and causes all sorts of problems on motorways and dual carriage ways..

John

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