Best way to confirm speedo reading.

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johnz3
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Best way to confirm speedo reading.

Post by johnz3 » Wed 06 Aug, 2014 20:10

I have a 3.0i sport and very quickly I can get up to around 100 mph. I however do not believe it is that speed. Any ideas apart from comparing/keeping up with a friend how I might check my speedo easily.

Jonco
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Re: Best way to confirm speedo reading.

Post by Jonco » Wed 06 Aug, 2014 20:27

Have you got a satnav or can you borrow one with speed indication?

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Mint
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Re: Best way to confirm speedo reading.

Post by Mint » Wed 06 Aug, 2014 20:33

I have a TomTom and when my speedo reads 70 I'm actually only doing 65.
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johnz3
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Re: Best way to confirm speedo reading.

Post by johnz3 » Wed 06 Aug, 2014 20:34

Good point re the Tom Tom. I will ask around. Thanks

Jonco
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Re: Best way to confirm speedo reading.

Post by Jonco » Wed 06 Aug, 2014 20:39

Mint wrote:I have a TomTom and when my speedo reads 70 I'm actually only doing 65.
I have the tomtom as well and there is a 3mph difference at 80mph. Speedo being higher on both cars I use it on. On recent road trip through France I passed several roadside speed indicators and they always agreed wth Tom Tom - I agree you do get some gps signal interruptions which temporarily distort reading.

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Re: Best way to confirm speedo reading.

Post by Kondor » Wed 06 Aug, 2014 20:40

Mint wrote:I have a TomTom and when my speedo reads 70 I'm actually only doing 65.
As far as I'm aware, this is standard with all car manufacturers, who allow some overestimation. Imagine the lawsuits if the odometer underestimated...

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Mint
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Re: Best way to confirm speedo reading.

Post by Mint » Wed 06 Aug, 2014 20:43

Jonco wrote:I have the tomtom as well and there is a 3mph difference at 80mph. Speedo being higher on both cars I use it on. On recent road trip through France I passed several roadside speed indicators and they always agreed wth Tom Tom - I agree you do get some gps signal interruptions which temporarily distort reading.
Yes, roadside speed indicators I've passed agree with Tom as well.
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Davejue1
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Re: Best way to confirm speedo reading.

Post by Davejue1 » Wed 06 Aug, 2014 21:14

I am a lorry driver and run on a highly calibrated electronic tachograph. My truck is restricted to 56mph and when doing so my Tom Tom is telling me I'm doing 54.If I was to be pulled over by the police or VOSA they would not be interested in me saying my Tom Tom was saying I was only doing 30 officer! The reading would only be taken from my tacho. So in short, don't rely on your Tom Tom as true because the likely hood is that it won't be. :)

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TR4man
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Re: Best way to confirm speedo reading.

Post by TR4man » Wed 06 Aug, 2014 21:29

Davejue1 wrote:. So in short, don't rely on your Tom Tom as true because the likely hood is that it won't be. :)

But a satnav is the best an "ordinary" motorist will get to accurately knowing their speed, unless they regularly have their speedometers calibrated.

I'm also under the impression that modern speed cameras also use some form of GPS so I'd suggest a satnav is more than accurate enough. - ignore this sentence, I'm writing rubbish !!
Last edited by TR4man on Thu 07 Aug, 2014 09:27, edited 2 times in total.

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Re: Best way to confirm speedo reading.

Post by gIzzE » Wed 06 Aug, 2014 21:51

Sat nav is accurate as long as you are not changing elevation at all. :)

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pingu
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Re: Best way to confirm speedo reading.

Post by pingu » Wed 06 Aug, 2014 22:28

gIzzE wrote:Sat nav is accurate as long as you are not changing elevation at all. :)
A sat-nav calculates speed by measuring the time between two "fixed" points. The most significant error is the error in fixing the points.

The more satellites, the more fixes and the more accurate the fix.

I don't know what the sample rate is for TomTom, but it must be quite rapid (>1 per second). Unless you are falling out of a plane, I don't think the error through elevation changes would be significant compared to your horizontal speed.


As said, I don't think Joe Public has access to a better speedometer than a sat-nav.
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Robert T
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Re: Best way to confirm speedo reading.

Post by Robert T » Wed 06 Aug, 2014 22:40

TR4man wrote:I'm also under the impression that modern speed cameras also use some form of GPS so I'd suggest a satnav is more than accurate enough.
Now I'm scratching my head at that one. GPS is used to tell you your position on the earth. By comparing two successive positions a fixed time period apart, you can calculate your speed. A speed camera is static. It knows exactly where it is at all times. :lol:

Static speed cameras use radar, laser or strips embedded in the road. Average speed cameras simply read your numberplate and record the start time, then another camera further up the road reads your numberplate again and records the finish time - the cameras are a known distance apart and so can calculate the average speed over that distance. The only kind of speed detection device I can think of is one fitted to a traffic car, which can use GPS to calculate the vehicle speed and radar/lasers to calculate the relative speed of the vehicle being followed.

GPS is reasonably accurate in a straight line over a reasonable distance. If it loses contact with the satellites, due to trees, buildings etc, then it can lag quite some way behind. Also due to only sampling the position every so often, then if you are accelerating or braking, then it won't reflect your true speed, as it can only calculate your speed say once a second.

I'd agree though that it is the most accurate way of determining speed that most of us have access to. You can do it by using the odometer to measure a known distance (say a mile), but it's not terribly easy to do and you need an accurately measured distance to do it. Other than that, it is a case of counting the turns of the speedo cable (or in the case of the zed the number of pulses from the optical pickup on the diff) for a set distance and scaling it up, but I don't fancy that much either.

I am actually going to have to do this shortly for my Sprite, as I have an electric speedo coming (so I can get rid of the speedo cable currenly draped in front of my seat) and I will be able to recalibrate it in the car. I am intending to use my satnav to confirm my speed. I'll let you know how I get on.

Cheers R.

Edit: For the record, I typed that before I saw pingu's answer. LOL.
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johnz3
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Re: Best way to confirm speedo reading.

Post by johnz3 » Thu 07 Aug, 2014 07:23

Would it be fair to say that if through the Tom Tom method my speedo was accurate at say 40mph it would be accurate at say 60 and 80 pmh. Thanks

Mike Fishwick
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Re: Best way to confirm speedo reading.

Post by Mike Fishwick » Thu 07 Aug, 2014 10:54

Most speedometers are progressively less accurate as speed increases, so jusy because yours is accurate at 40 mph don't rely on it at higher speeds, where it will read high.

The BMW speed sensor is actually a magnetic reed switch - it connects the signal wire to earth or not, to produce a negative-going square wave.

However good your electronics are, the analogue speedomenter has the usual type of errors inherent in such devices, and is permitted a maximum legal error of 10% plus and 0% minus indicated speed. This seems to be rather high, as the British Standard for analogue meters in industry is, or was, 1.5%.

Quite apart from that, the best speedometer is only as accurate as its original calibration, and even the official speedometers on police vehicles have to be recalibrated when new tyres are fitted, or the tyres are half worn, and also annually.

Although they are pretty good, a GPS receiver cannot always be relied upon as being legally accurate, leaving the only really accurate means of checking your speedometer - or GPS - as timing between three of the kilometre posts on a motorway - that is, two kilometres or 1.25 miles. In round figures, 60 mph this distance will take will take 75 seconds, at 70 mph 63 seconds, and at 80 mph 50 seconds.

I found that my speedometer was almost 5 mph fast at an indicated 80 mph, and about the same at 70 mph.

I'm sure it would be possible to produce an accurate speedometer, but I bet the manufacturers are told by the government to introduce a certain level of optimism, as a road safety feature, and to keep the customers happy.

The French seem to have good errors on their speedometers, as when in a 30 mph zone they all slow down to about 25 mph, and around 37 mph in a 45 mph limit. Incidentally, I know of a French speed indicator on the D710 near Perigeaux which is about 5 kph slow - and it is just before a camera!
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littlefeller
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Re: Best way to confirm speedo reading.

Post by littlefeller » Sun 10 Aug, 2014 08:10

assuming you have the correct size wheels fitted, most bmw gearboxes have a 1 to 1 ratio in 5th, your tacho needle and speedo needle should rise in a linear fashion (assuming the tacho is accurate :lol: ). for instance - speedo needle is pointing dead vertical then the tacho should be exactly the same. tom tom is the best poor mans gauge, not all speedos read under, my last car - Honda, was the other way around on original wheels - tom tom 60mph car speedo 56mph :shock: it got worse as the speed increased simply because its a percentage (a percentage of a slow speed makes for a smaller difference than a higher one, if you calculate this difference as a percentage at a slow speed then apply it to a higher speed I have always found it to be cock on to my prediction)

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Re: Best way to confirm speedo reading.

Post by littlefeller » Sun 10 Aug, 2014 08:55

you could build yourself a spread sheet like mine, this is assuming your tacho is acurate :lol:

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Mike Fishwick
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Re: Best way to confirm speedo reading.

Post by Mike Fishwick » Sun 10 Aug, 2014 10:20

While it is easy enough to calculate what the speed should be for every 1000 rpm in top gear etc, what allowance will you use to take into account the inevitable errors in the instruments?

Why do you assume that a tachometer is a standard by which to judge a Speedometer? OK - the speedometer has its largest error from circumference changes caused by tyre wear and/or the use of non-standard rear tyre sizes, but both use the same type of instrument to display their information.

A simple distance and time calculation will give an accurate speed, which will then tell you the speedometer and GPS error at that speed. Checks at other speeds will enable you to know the speedometer error at a range of speed limits. Knowing your speedometer error will also make it possible to calculate the tachometer error.

On the subject of GPS, remember that when the US was involved in the Gulf War they moved a lot of satellites closer to the Gulf area in order to provide a better fix for cruise missiles, at the expense of extreme accuracy over Europe.
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Davejue1
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Re: Best way to confirm speedo reading.

Post by Davejue1 » Sun 10 Aug, 2014 11:41

You will not get much Better than a lorry tachograph as these are used to prosecute drivers all the time by VOSA and the police. That said they are re-calabrated regularly and when a new set of drive tyres are fitted. After this thread fired up last week I did check again on my TOMTOM Speed against the tachograph. I did this on two different lorrys and on both occasions the TOMTOM was reading slower than the tachograph. This is my 3rd TOMTOM since they wear out as I use them daily and none have read the same as the tachograph. I would not trust a GPS device over a tachograph when it came to keeping my licence. :)

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Robert T
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Re: Best way to confirm speedo reading.

Post by Robert T » Sun 10 Aug, 2014 13:51

Just to throw another can of worms onto the fire...

The vehicles idea of speed may well be different to what is shown on the speedo. This is because the vehicle speed is measured digitally and then converted to an analogue display for us fleshies to read. You can put the odometer into self test mode to get a speed readout in km/h - it would be interesting to know if 80 km/h does actually show as 80 km/h on the speedo. If you are going to put in a deliberate error of say 5%, then that is where it could be done, whilst maintaining an accurate distance measurement for the odometer. If the error is in the sensor measurement, then not only is your speedo going to be out, but your odometer reading is too!

I'll give this a try when I get a chance, but if anyone else wants to try it, then details of the odometer self-test mode are in the knowledgebase.

Cheers R.
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Mike Fishwick
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Re: Best way to confirm speedo reading.

Post by Mike Fishwick » Sun 10 Aug, 2014 15:51

Even using the digital speed readout, it is only as accurate as the number of pulses per kilometre - and that depends on the circumference of the tyres, which normally varies with tyre wear and to a small degree the size difference between the factory optional wheel/tyre combinations. Once people start to fit larger size wheels and tyres this size difference can increase, so reducing the number of wheel rotations per kilometre and making the speedometer read low.

Anyway - I doubt if the digital-to-analoque conversion circuit will convert the number of pulses per per mile with absolute accuracy.
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Re: Best way to confirm speedo reading.

Post by Brian4 » Sun 10 Aug, 2014 20:43

I think Johnz3 was really asking if his speedo is correct at 100 mph or is he doing a lot less so would think the TomTom idea is his best bet. The technicalities of speedo calibration aren't in his original question, just trying to get back to the 1st question.
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Mike Fishwick
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Re: Best way to confirm speedo reading.

Post by Mike Fishwick » Mon 11 Aug, 2014 13:45

I thought we had all been replying to the original question - how can a speedometer reading of 100 mph be checked for accuracy? The answers were:

(a) Comparison with the tachometer, based on the usual Z3 25 mph/1000 rpm in top gear, so giving 4,000 rpm at 100 mph.

(b) Comparison with a GPS speed readout.

(c) Timing over an accurately marked distance.

(d) Comparison with the digital km/hour readout.

As discussed, all these methods have various potential for error, but I thought we had given food for thought.
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Robert T
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Re: Best way to confirm speedo reading.

Post by Robert T » Mon 11 Aug, 2014 15:05

100 mph ought to be easy. Just drive past a speed camera and see what it says on the ticket. :wink:

Cheers R.
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Re: Best way to confirm speedo reading.

Post by Davejue1 » Mon 11 Aug, 2014 16:11

Nice Robert, very nice!! :D
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pingu
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Re: Best way to confirm speedo reading.

Post by pingu » Mon 11 Aug, 2014 22:47

Robert T wrote:100 mph ought to be easy. Just drive past a speed camera and see what it says on the ticket. :wink:

Cheers R.
If you are lucky (like I was), it will say 99.5mph :wink: .


BTW, my speedo was reading 110, but it wasn't a Z3, so it's allowed to be wrong.
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Re: Best way to confirm speedo reading.

Post by littlefeller » Tue 12 Aug, 2014 07:18

speed guns are notoriously inaccurate, all measurement devices are subject to a margin of error. I think I would rather have a speedo that reads slightly over, but I think the original question is - how big does this inaccuracy get the faster you go. 10% of 30 mph is 3mph but 10% of 100 is 10mph so it just gets bigger, I would factor in around 10%. this seems to be close if you drive past those road side speed displays (again how were these calibrated) but yeh around 2-3 mph at 30 I see quite often.

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Re: Best way to confirm speedo reading.

Post by alec.m » Tue 12 Aug, 2014 15:32

I`ve done a comparison on mine against my sat nav and it`s about 4mph out at 20mph and the same at 80mph so I don`t think it`s a percentage thing, more where the dial is set against the needle when assembled.

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Re: Best way to confirm speedo reading.

Post by Mr Silver » Tue 12 Aug, 2014 16:37

^ About right Alec. Unlike the cable drive instruments of old the speedometer in the Z3 is, I am led to believe, essentially a rotary servo. For those unfamiliar with this beast it is a device that converts serial digital data into a rotary position. Due to its method of operation it can't really give an incorrect reading (relative to the input data stream) but the vehicle manufacturers always put in an offset, that is, the speedometer must read a percentage high. This is achieved by positioning the needle slightly too high (clockwise) on the shaft. The end result is that the error, typically 4%, is constant throughout the range of the instrument.


Apologies for the detail, hope it helps.


Regards.

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Re: Best way to confirm speedo reading.

Post by Mike Fishwick » Tue 12 Aug, 2014 17:26

The Speedometer is NOT a servo system! Servo systems operate by comparing a reference value (whatyou want) with a feedback signal (what is really happening) and generating a difference signal to bring the two into alignment. That would require the instrument to give a position feedback sigal to a system which could corect the indicated speed.

all we have is a digital output from the sensor in the final dirve which is processed by a digital to analogue converter (frequency into voltage) to generate the necessary DC signal to operate the instrument - which can give an inaccurate a signal just like any other instrument.
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Southernboy
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Re: Best way to confirm speedo reading.

Post by Southernboy » Tue 12 Aug, 2014 18:18

Alec, if your speedo degree of error is 4mph at 20mph you're looking at around 20% error !! That is way over the "norm" which I have always believed to be around 5%. I do know that when my speedo shows 60kph, my Garmin warns me that I'm "speeding" by sounding a gong tone !! 60kph is the general speed limit here in urban areas.
I assume that the Garmin can't keep up with the constant recalculation, but even using my cruise control to maintain a set speed on a flat road will evoke the Garmin's disapproval. :D
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alec.m
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Re: Best way to confirm speedo reading.

Post by alec.m » Tue 12 Aug, 2014 19:55

Hi Barry, my point is that my speedo error is 4mph at whatever speed I`m doing so there is no constant percentage error. If I bent the needle slightly anticlockwise to make it accurate at 20mph it would then be accurate at 135mph. It`s just not set at zero properly. Hope that explains the way mine reads.

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Re: Best way to confirm speedo reading.

Post by littlefeller » Wed 13 Aug, 2014 06:33

explains it perfectly. personally I wouldn't know if the clock was out at speed as I don't anymore (getting old) fastest I have driven the z was passing a row of trucks (80mph), but I was in the isle of man :wink: so are these clocks then pretty accurate. :?:

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