OT: Car speedometer accuracy

Got a new car last week (3-yr-old Yeti). When I was using the satnav I noticed that the car's speedo was consistently 2 to 5 mph higher than the speed that the satnav showed.
I know that cars' speedos tend to be a bit inaccurate (even a 63-reg Yeti??). And as for the stanav, it needs to be seeing enough satellites (I guess) to get the speed correct.
So does anyone know which is likely to be *more* accurate? Speaking as someone who's just been nicked for going 35 in a 30 zone, I'd be interested to know.
Cheers John
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Another John wrote:

Sat nav will likely be correct Car Speedos must NOT underestimate and so to be sure they tend to over read by a few % in order to meet the specification.
You notice this in the Average speed zones, if you set the cruise control from the satnav, you are continually chasing up the arse of the car in front who is driving on his speedo
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On 30/10/2016 21:48, Bob Minchin wrote:

Not if you have adaptive cruise control...
(You are still behind them travelling slower than the limit, but you keep a decent distance from their arse.)
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wrote:

My van's adaptive cruise control and steering just undertakes them........
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Must not over read by more than 10%

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bert

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In these electronic days it would be trivial to make a speedo accurate.
The +10/-0% at 30 mph dates back from crude eddy current types where it would increase the cost to make one accurate.
It just suits some car makers to specify their speedos over read to the maximum allowed by law. If it were simply tolerance, then the error would vary in different examples of the same car (or make). But it tends to be extremely consistent.
Nor can you guess by price whether a maker will fit an accurate or optimistic speedo.
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On Monday, 31 October 2016 15:14:49 UTC, Dave Plowman (News) wrote:

I prefer to read stuff from those that actually know.
http://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/
Saving a lot of wasted time, discussing things with those that don't know.
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'To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount.'
Very good article that - and written just for you. No need for boring figures - 'a certain amount' is just fine. Very scientific.
But tell us of your findings, Dave. What is that certain amount on your car? And on all the others you've owned?
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On Monday, 31 October 2016 16:56:25 UTC, Dave Plowman (News) wrote:

No it wasn't why would it be.
>No need for boring

couldn't care less .

IU have no finding to report.
What is that certain amount on your

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On 31/10/2016 16:28, whisky-dave wrote:

<thecarexpert>
The differences in wheel diameter resulting from the above circumstances [eg wear] could be tiny (maybe a few millimetres), but at 30mph your car wheels are rotating 6-7 times every second, so it can quickly make a difference of a few miles per hour.
</thecarexpert>
"Those that actually know." Riiight.
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Just the sort of figures given by a journo who can't use a calculator. Perhaps Dave will use his and give the actual speed error at 30 mph of say a 4mm difference on a wheel turning at 6 times a second? Could prove interesting.
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On 01/11/2016 00:51, Dave Plowman (News) wrote:

Only for one based on GPS observations.

The "car expert" sounds like it is aimed at innumerate morons.

Nominal 220mm radius wheel a change of 4mm is a shade under 2% change in the indicated speed irrespective of the actual speed of the vehicle.
Indicated speed is typically more like 5% high as a default today. You can test a speedo against measured miles (now fairly rare), GPS or fixed radar your speed indicators (not always properly calibrated).
Is the one on Blackpool north cliff promenade still there?
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Measuring the time to travel a measured mile has an inherent error which needs to be taken into account: the reaction time of the person who is operating the stopwatch as it passes the start line and the end line. Also the driver needs to be able to drive at a constant speed without having to slow down as the come up behind a slower vehicle ahead - you need a quiet motorway and a cruise control (or a very steady right foot).
The error in timing over a measured mile could be around 2%: if you are travelling at 60 mph it will take you a minute to cover the distance so an error of 1 second will be 1/60 * 100 %.
I remember the very first time I travelled on an HST 125 train: my grandpa had drawn up a list of times between one 1/4 mile post and the next, and the corresponding speeds. When we were up to full speed I got out the stopwatch, timed from one post to the next, referred to his table and arrived at a speed of 140 mph. This led on to a long lesson from grandpa (he used to be a teacher!) about error and accuracy: that it was better to measure over a longer distance/time to reduce the timing error. I've never forgotten that lesson! What would he have made of GPS and the ability to measure speed directly - always assuming the carriage body/windows didn't block GPS signals!
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It was possible to make a speedo accurate (in terms of reading an analogue needle type display) long before GPS was thought of. And electronics make it very much easier today.
Speedos used to be called clocks. You'd laugh at any clock which was as inaccurate as the spec for a speedo - a 10% error allowed? Set it correctly at midday and it's over an hour out at midnight? They could do better than that hundreds of years ago.
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On Tue, 01 Nov 2016 12:21:22 +0000, Dave Plowman (News) wrote:

Well, the "Chronometric"(tm) speedo heads *were* actually based on mechanical clockworks, not a magnet or aluminium disk in sight. They literally measured a distance over about a quarter of a second gating interval derived from a balance wheel escapement mechanism.
The speedo cable feed drove a clutch disk which in turn rotated another disk against a restoring spring which would return it back to its reference starting point. This disk had a peg which in turn drove the indicator needle around the speedo dial against a detent which would hold the reading during the next measuring cycle. The detent would be released briefly at the end of the subsequent measuring cycle before locking the reading so that on deceleration, the needle could drop back to the slower speed measurement. During acceleration, the needle would simply be nudged up to the higher reading every quarter of a second or so.
These could be calibrated very closely to the actual road speed by adding or removing clip on shims to trim the mass of the balance wheel and were guaranteed to be linear (give or take a detent notch - 1mph I believe).
However, the real issue with speedo accuracy derived by measuring the prop shaft or road wheel revolutions has more to do with the differences between new and end of life tyres (where, fortuitously, the effect of wear on the tread blocks which results in a reduced tyre diameter is countered by the reduced give or 'slip' in the now reduced profile tread blocks), the effects of "Tyre Growth" at high speeds due to centripetal force and the variations in 'slip' due to variations in wind resistance and slope of the road.
Whilst the basic spinning magnet and aluminium disk speedo can't be so readily calibrated to the same accuracy as the classic "Chronometric"(tm) speedo heads, with care, it is possible to approach this level of accuracy very closely but, when all the other sources of variations in the rest of a system dependant on sensing speed from the prop shaft or road wheel(s) are taken into account, there's not a lot of point in doing so. It is sufficient that a given model of speedo head intended for any one model of road vehicle remains consistently matched to the rest of its siblings coming off the production line.
Modern satnavs can generate quite precise average speed over ground readings which, in conjuction with maintaining a steady cruise speed derived from the vehicle's own speedometer, approximates very closely to an actual road speed for comparison against the speedometer from which you can derive your own personal calibration curve by which to assess true speed versus the speedometer reading.
For any number of reasons, it should come as no surprise to find a modern car speedo is likely to be some 5 to 10 percent optimistic of the actual road speed it purports to be measuring. All you're doing when 'calibrating' the speedo from your satnav's average speed readings, is determining the magnitude of this error for future reference, bearing in mind that it's probably best to assume the speedo reading as 'accurate' for steep downhill stretches of high speed sections, especially when enjoying the speed enhancing effect of a strong tailwind into the bargain.
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Johnny B Good brought next idea :

Thanks for that explanation, I did wonder how they worked.
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Yes. Characterised by a stepping movement. However, eddy current ones are pretty consistent. So if properly calibrated in the first place can be perfectly accurate enough. Certainly far better than a 10% error at 30 mph.

Right. It's one of the few things cars wise I've not owned or had apart.

As has been said, tyre wear can only result in the speedo over-reading even more than the law allows. But never under read. Tyre growth is going to be minimal at the sorts of speed legal in the UK.
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On 02/11/2016 00:23, Dave Plowman (News) wrote:

Maybe but maybe not. Don't forget they fit the same speedo to several models and they change the wheel and tyre sizes depending on the trim level so the inaccuracies are going to be quite high.
With digital ones they can change the parameters to match which is why they can be more accurate unless the dealer has done a special and swapped the wheels/tyres.
The driver fitting winter tyres may well cause errors too as they are unlikely to be the same size.
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Plenty models share the same look speedo - but it is individual to the actual model. If they can decide which size wheels to fit surely they can pick the correct speedo from the bins? And models with a choice of engine sizes etc in the same body will likely also have different final drive ratios.

Can they? I've not actually seen speedo programming being part of software choices. Not to say it isn't.

Fitting the incorrect wheels and tyres could alter the way the speedo reads either way. So is no reason for not to have it reasonably accurate in the first place. An up to 1% error on new tyres would be easy to achieve.
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On 02/11/2016 13:44, Dave Plowman (News) wrote:

why would they when it adds cost?

Winter tyres and wheels are not incorrect.
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