I know that there has been a lot of discussion previously regarding the inaccuracy of the speed indicated to that actually being done (c. 5-10% IIRC).
However I was doing an indicated 90 mph and put oin cruise control. I then checked the average speed that I was doing on the trip computer and it said that I was doing 86.3mph.
Which one is likely to be more accurate or will they both be wrong? If the trip computer is going to be the correct one, why not have a digital speedo?
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Personal view - the trip computer would be more accurate as it is probably electronic as opposed to mechanical.
A kind TraffPol once calibrated my speedo for me. I was on cruise control at steady 120 on the clock, and was VASCAR'd at 108 - exactly 10% wrong on the speedo (well, depending how you do your sums). He gave me a ticket to prove the calibration too, and it was all verified legally in court etc.
Still, I got off with 2 week ban and £180 fine...could have been worse.
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What no 'points'? You were very lucky IMHO
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I used a good local solicitor - about £200 - I think that helped. Found him through teh Citizen's Adice Bureau. He also got me into court within 30 mins of 10 am, instead of having to wait around all day which was nice.
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Speedometers should always over-read, it's a legal requirement. Indicated speed should be ((0 to 10%) + 4kph) above acutal speed. Trip computers will normally display vehicle speed accurately.
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Why don't they replace the speedo with the trip computer then!?
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Too easy to fiddle, I believe.
Electric haircut, sir?
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It is not a legal requirement to overread;the legal requirement is not to underread;the speedo must fall within 0 to 10% high and this is checked at three speeds and I can't remember what they are.
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You're right about not under-reading, but the regulation is that the speedometer must read 0 to (10% + 4kph) above actual speed. This should be tested at three speeds - and these depend on the max speed of the vehicle.
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Do we have any keen mathematicians who would like to calculate the difference to speedo readings between new tyres and worn-to-the-limit tyres?
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If you search Dizzy, you should find its been done. It was in the old backroom about 4 months ago or so.
If I have time later I'll try and find it. If you find it before me, perhaps you'd put a link in this thread, or let me know and I'll transfer the old thread here.
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Dizzy
Easy enough. If you wear 5 or 6mm off the tyre (this will get most down to 2mm) on an average size tyre the circumference changes by 1.6 to 2 %, which represents the speedo error. Obviously it will read fast as the tyres wear.
Regards
John S
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Using JW's criterion, Smokie's speedo was out of spec.:
(120-108)/108 x 100% = 11.1% high
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It's funny, for the years since that offence I've been thinking exactly 10% but of course you are right - the percentage over-read is greater than 10%.
Lucky I wasn't done again then, as I drove on the basis that it was exactly 10% wrong from that point forwards (well, mostly ;-)
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You're right that it read 10% wrong w.r.t. the displayed value !
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I think the Smokie's speedo was just within spec. If he was doing 108 mph the speedo could read between 108 mph and 121.3 mph (108 + 10% + 4kph). Assuming my maths is right!
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I think the Smokie's speedo was just within spec. If he was doing 108 mph the speedo could read between 108 mph and 121.3 mph (108 + 10% + 4kph). Assuming my maths is right!
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Well I won't argue anyway. I sold the car three + years ago.
To move on, I failed to complete my bit of the form, and subsequently wrote to the DVLC, not knowing to whom I had sold the car. I was grateful to myself for my foresight, as the car picked up two speeding and one parking offence, which were notified to me, but when I pointed out the car was no longer mine I heard no more.
Except some months later when I got a letter from a recovery company saying it was abandoned and if I didn't collect within 4 weeks with £100 they would destroy the car. I called them to find out what was wrong with it, they weren't sure but "it wasn't a runner". So I let it go.
I liked it too...Senator 2.6 auto.
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Ah, yes, I missed the 4 kph bit
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The first two speeds are the same on all vehicles;the third is dependent on max. speed.
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It is not a legal requirement to overread;the legal requirement is not to underread
Wrong! The speedo *must* over read.
Of course it doesn't need to overead by much.
I'm gonna set the cat among the pidgeons here and claim speedos are pretty accurate.
Evidence?
a) On a known 119mile car/bike ride home the odometers of two cars and on bike always display as near as damn it 119. Never more than a mile off.
b) RIding with mates in 30,40,50 lmits our speedos *all* tally. (Except one guy who I suspect hasn't learned to read it properly...)
Based on this either a load of random speedo's are all wrong by an identical amount or they're pretty accurate.
--
Parp, Parp!
Note: All Toad posts come with an implied smiley.
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The speedo must overread?-it could be 100% accurate.However manufacturers may offer more than one tyre option on a car(at one time the Granada/Scorpio had twelve different tyre/wheel combinations offered-even Escorts had five or six-tho'in some cases a different speedo gear had to be used) and if the rolling radii are slightly different they will choose a combination of speedo drive gears(mechanical or electronic) that will allow the speedo readings to fall within the zero to +10%; most will be in the 0 to +5% region.The relevant EC directive says speedos must not underread-not me.
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The speedo must overread?-it could be 100% accurate.
A traffic rozzer I know says in *must* overread. (But as you'll see from my other post I don't see why it should over read by much)
>>The relevant EC directive says speedos must not underread-not me.
Maybe my traffic rozzer contact is wrong but he was adamant about it.
--
Parp, Parp!
Note: All Toad posts come with an implied smiley.
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I was stopped by an adamant copper who quoted the law to me incorrectly and refused to go back and look at roadmarkings that would have proved me correct.
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It is not a legal requirement to overread;the legal requirement is not to underread
Wrong! The speedo *must* over read.
The FIRST statement is correct.
AND the tolerance is -0 to plus (5% plus 4 kph).
This results in a maximum legal display of 34 at 30.
Hence the popular "10%" misconception.
This is, of course, as manufactured.
Whether anybody would like to guess a reading from a Morris Minor speedo is an altogether different issue.
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Wrong! The speedo *must* over read. Evidence? a) On a known 119mile car/bike ride home the odometers of two cars and on bike always display as near as damn it 119. Never more than a mile off.
The odemeter would be more accurate than the speedometer because the odometer is gear driven, whereas the speedometer is magnetically driven. Depending on the strength of the magnets (age depreciation), and the tollerance gap allowed between the magnets, there is room for error.
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So is the trip computer's speed reading taken from distance/time?
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In my experience, yes. The odometer is more accurate than the speedometer.
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Many of today's speedos are not magnetically driven but by an output from the ECU(engine control module) which has inputs from the engine and gearbox.
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I know that some manufactuers take vehicle speed signal from ABS units - since the ABS unit has a wheel speed sensor on each wheel to allow braking control. This signal is then passed to the speedometer and the engiine control unit and anything else that may need it. If ABS isn't fitted than a sensor on the gear box is used.
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A quick way to test your speedometer.
On a Motorway the Marker posts are set at half furlongs, 16 to the mile.
Keep the needle steady and get your passenger to time over a mile
With needle steady at 50mph = 1min 12 secs
60mph = 1 min dead
70mph = 52 seconds
(if I remember correctly)
Any deviation over 2 secs plus or minus means possible fault.
Use the same to check odometer.
DVD
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One other way to check the accuracy of your speedo is to beg, borrow, steal or buy a handheld GPS receiver. It will tell you your speed over ground with uncanny accuracy, updated every second or so, making it easy to compare with the speedo (preferably with someone else driving). Mine showed that the trip computer on my Audi was spot on, but the speedo was reading 5% fast.
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Much of the discussion above seems to centre on semantics, so I thought you might like to know what the regulation has to say. The regulation in question is 75/443/EEC, section 4 being concerned with the accuracy test.
The regulation requires the speedometer to be tested at 40, 80 and 120 km/hr (if the vehicle cannot do 150km/hr or more then the upper test speed is 80% of the vehicles maximum).
With regard to accuracy the exact words of the regulation section 4.4 are: "the speed indicated must never be less than the true speed". There is then an equation for calculating the limit of accuracy. Since I cannotreproduce this equation here in mathematical form what it says in words is that the upper limit is 110% of the true speed +4km/hr. The lower limit is 100% true speed. The accuracy may fall off outside these limits above
120km/hr.
So according to the letter of the law it is not true that the speedo must overread, since it can be 100% accurate. However for all practical purposes this statement is essentially true, since it would not be possible for any manufacturer to mass produce 100s of 1000s of cars per year all of them with 100% accurate speedos. Most manufacturers will take into account manufacturing tolerances, different sized tyres & tyre wear and design their
speedos to fall in the +5 to 8% range nominally.
Most electronic speedos these days are stepper motor driven, so the accuracy usually remains roughly the same at all speeds (in %age terms). Older magnetically driven types usually got less accurate as the speed rose (and with age).
Trip computers are entirely electronic and not subject to the above legalrequirements so manufacturers can set them to be 100% accurate nominally. The odo is usually motor driven too, but is similarly excluded from the legal requirement and so can be set to be 100% accurate nominally.
If anyone wishes to read the regulation for themselves, please email me.
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dont cars like the yaris with digital speedo's over-read also ?
i'd be surprized if they didnt, certainly feels like it driving one
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must do - they are subject to the same legal requirement as any speedo, though one would suppose the ability to read acurately should be greater.
Interesting useless info point: when NASA were developing the shuttle, they spent millions researching the type of instruments to use. They determined that the human brain makes much more sense of analogue instruments than it does of digital readouts when the information is required to be assessed quickly. This is why the main instrumentation in the shuttle is analogue and even the latest planes use electronics to display an analogue representation of an instrument. I would have told them that for just a couple of hundred thousand bucks!!
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same as all of the research into what kinds of personalities make the best submarine crews, locked together potentially for months on end
turned out that a balanced cross section of ordinary personality types was the best
of course thats right cos thats how nature chose us in the first place ?
still think the emergency repairs to the swiches on the moon landers etc are such great stories
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