Speedo/odometer inaccuracies

[davenewellhome]

As some of you know we've just been to the Dusseldorf caravan Salon. I used a VDO Dayton PC5100 sat nav which has a trip computer facility. I zeroed the trip computer before we left and the final readings make quite interesting viewing. We covered a total of 999 miles according to the nav cmputer, which is within 5 miles overall of the estimated mileage for our route as per the Via Michelin website. We used my wifes 2001 Ford Focus 1.8 diesel car whose odometer recorded 1018 miles, a difference of 19 miles or 1.9%. The sat nav gives an accurate speed reading and from this we could easily see that the Ford speedo reads high by 10%.

 

Now I find this a little unusual in that the two parts of the same Ford instrument (speed and mileage recording) give two different error percentages, 10% over-read for the speedo, which is on the top limit legally but only 1.9% over-read for mileage.

 

If anyone else has a sat nav the features a trip computer I'd certainly be interested in what percentage errors your vehilce gives for speed and mileage.

 

Other than that I was very pleased with the Ford, it's supremely comfortable, even for me with my dodgy back and sciatica problems and we averaged 55MPH according to the nav trip computer. Oh yes, air con is such a godsend!

 

D.

[colin]

Supprised you only got 1.9% overread on milage, roumer would have it that most milage meters overread by 10%, the more sinical would have it this was to make you think you where getting better mpg and to bring up service earliar.

Got to agree with you on Focus, much better than Astra of that age, also for getting in and out, apparently they dressed up in 'old people' suits to get better ergomomics.

[davenewellhome]

Likewise my surprise Colin. don't care who they dressed up as what, that's one of the best long distance drives I've ever done! (long distance driving is not new to me, I've done this stuff before).

 

D.

[docted]

Dave I believe that the difference is due to the distance up and down hills which the sat nav has problems in computing because it does not actually see the hill and vertical distances only the horizontal distance covered.

 

Docted

[Dave Newell]

Sorry Docted that is just not true. GPS is very accurate and does measure altitude as well, it is in fact a three dimensional location system. Besides which the error should be the same for both if it were down to the GPS and that is my point, the speedo and the odometer errors are different by quite a large margin.

 

D.

[Old Git]

Dave Newell - 2007-08-27 9:46 AM

 

Sorry Docted that is just not true. GPS is very accurate and does measure altitude as well, it is in fact a three dimensional location system. Besides which the error should be the same for both if it were down to the GPS and that is my point, the speedo and the odometer errors are different by quite a large margin.

 

D.

 

Very interesting Dave. It seems quite contradictory that the same instrument is inconsistent in two closely inter-related readings.

 

I had also noticed that my speedo shows a higher reading than the GPS trip meter, and by about 10% - I'll take a more careful note next time out and post you the details. I have never compared the recorded distances on GPS and odometer however, but again will do so next time out.

 

Without a fairly accurate table of data from a considerable number of sources you will never be able to reach a reliable conclusion. Might I suggest that you post a short list of the data you would like and ask members to send in their observations. If there is a pattern you will then get a fairly accurate picture of what it is - though it might be a bit more difficult to find a reason.

 

How do modern speedo/odometers work? The anomaly could be explained quite easily with the old models, as the odometer mechanically counted gearbox final drive revolutions, but the speedo relied upon magnetic inductance (think I'm right here) to drag the needle round and indicate speed. Distance could therefore be assumed to be accurate, but speed was only ever a calibrated approximation.

 

I suspect the modern ones operate digitally in all readouts, and this makes the inconsistencies far more difficult to explain, as both speed and distance would be measured by recording the number and/or frequency of pulses. Both should be spot on - in theory at least.

[devonchris]

Just a mad second of thought will a gps system take into account the passing of parked vehicles, overtaking, and roundabouts (using the outside lane) ect.

 

 

chris

 

 

 

 

 

 

 

[Dave Newell]

Dave, (old git) you're just about spot on with how speedos did and do work. Until recently the speedo was still driven from the gearbox output but via a pulse generator and electronic signals. More modern vehicles are using an averaged output from the ABS controller to give a speed signal and the speedo head is electronically driven. The speedo on this Ford reads over by ten per cent across the range, well from as slow as you can measure a difference anyway.

 

Devonchris, don't get hung up on the GPS technology, the issue here is the difference in error from speedo to odometer but in answer to your question GPS systems generally take a location about once per second and then calculate distance moved and direction as a straight line between current and last point, speed is a simple calculation of distance moved against time. So your GPS journey is a series of straight lines between measured points. As your speed increases the actual distance between points increases so yes, technically there could be a discrepancy but its not going to be anything like 10% and it certainly doesn't account for the difference in error percentage. Unless of course you regularly scream around roundabouts at 70MPH (lol) .

 

D.

 

[tonyishuk]

Total silly question which I will try on my Tomtom later,

 

Is the distance around the M25 same in the clockwise direction as the anticlockwise direction ?

 

There is a diiference , but unable to find out what it is (Clockwise is longer by a couple of miles or so, I believe)

 

Rgds

[davenewellhome]

tonyishuk - 2007-08-27 5:45 PM

 

Total silly question which I will try on my Tomtom later,

 

Is the distance around the M25 same in the clockwise direction as the anticlockwise direction ?

 

There is a diiference , but unable to find out what it is (Clockwise is longer by a couple of miles or so, I believe)

 

Rgds

 

You're probably right but what has that got to do with this subject please?

 

D.

[enodreven]

Hi, Dave

 

I know you say that the speedo was consistently over by 10% but was this even at very low speeds e.g. when pulling away stopping etc as you would have had to have been watching very very closely even if you could see any reading?? as my speedo dosen't appear to start until the car is rolling and in fact you are only looking for 8% ?? don't you think this could be a possible inclusion for the discrepancy, also did you lose the GPS signal at any time as we do quiet often albeit it locks back on automatically ?

 

Just some thoughts hope that helps

Brian

[trooper]

HI' some speeedo's are very inaccurate.

While in france with some friends, we decided to check his suspect speedo,

we had radio's so we could check acurately, At 70 mph on my speedo his was 60 mph, also he was running at 3000 rpm at this speed while I was running at 2250, down through the speeds there was the same discrepiancy, this was the final straw for him as his m/h had spent more time at the dealers than at home, guess what ? it was a hymer A class.

Cheers Don

[hymer]

My Hymer Exsis 2.8 reads 15% out

[davenewellhome]

Perhaps I didn't make my point very clearly, I'm not bothered about the inaccuracy and I don't particularly need to know why there is a difference between speedo and odometer. It was merely an observation and, if you like, an open invitation for anyone else who can to check theirs and share their results.

 

Brian, the speedo is 10% high from an indicated 20 MPH (18MPH true speed) up to an indicated 80MPH (72MPH true speed) so no it doesn't explain the difference.

 

If anyone else has the facilities to do a similar comparison you need to do it over a sensible mileage of at least several hundred. I'm interested in total mileage covered according to odometer and GPS as well as the percentage error of your speedo. This is not a particularly scientific study, just a small matter of personal interest.

 

D.

 

 

[tonyishuk]

davenewell@home - 2007-08-27 6:42 PM

 

tonyishuk - 2007-08-27 5:45 PM

 

Total silly question which I will try on my Tomtom later,

 

Is the distance around the M25 same in the clockwise direction as the anticlockwise direction ?

 

There is a diiference , but unable to find out what it is (Clockwise is longer by a couple of miles or so, I believe)

 

Rgds

 

You're probably right but what has that got to do with this subject please?

 

D.

 

Broadly how acurate are SatNav distances, when measuring the same route, when there is a know difference, and if that difference in those distances vary from the actual distance that is measured by other means,

 

The M25 is a a documented length, and I was wondering how a Sat NAv differed.

 

I was following a slightly diverse but similar topic that was under discusssion I thought :-)

 

regds

[davenewellhome]

Thanks Tony. As far as I'm aware the only topic of discussion on this thread is the apparent error difference between the speedo and odometer on my wife's car. There will be a slight difference between GPS measured distances and true distance covered for the reasons I gave above but unless you go round very tight corners very quickly as a matter of course then the difference will be very low. As I've said earlier this would not account for the difference between the speedo and the odometer. As they are both driven from the same signal it would seem natural, at least to me, to expect them to display the same error.

 

D.

[JayKay]

Hi Dave,

 

Have you lost the will to live yet, if not perhaps I can add my little bit.

 

Citroen Relay works van:- Garmin sat nav + Road Angel shows 6% error on van speedo.

 

Ford Focus 2.0 Estate:- Road Angel shows 10% error.

 

Hymer A class Fiat base:- Garmin sat nav + Road Angel shows 5% error.

 

All vehicles speedo's are reading above true by the stated amount.

Sat nav & Road Angel both showing same speed.

 

I purchased the "Road Angel" about two years ago and I noticed the same as you. On a trip to Hartlepool we tried the Road Angel in friends all singing, all dancing VW Passatt and got an 8% error on his speed

 

I hope this has not clouded the waters and helps in some way.

 

John D.

[Dave Newell]

JayKay - 2007-08-28 2:28 PM

 

Hi Dave,

 

Have you lost the will to live yet, if not perhaps I can add my little bit.

 

Citroen Relay works van:- Garmin sat nav + Road Angel shows 6% error on van speedo.

 

Ford Focus 2.0 Estate:- Road Angel shows 10% error.

 

Hymer A class Fiat base:- Garmin sat nav + Road Angel shows 5% error.

 

All vehicles speedo's are reading above true by the stated amount.

Sat nav & Road Angel both showing same speed.

 

I purchased the "Road Angel" about two years ago and I noticed the same as you. On a trip to Hartlepool we tried the Road Angel in friends all singing, all dancing VW Passatt and got an 8% error on his speed

 

I hope this has not clouded the waters and helps in some way.

 

John D.

 

Thanks John but did you note the odometer readings and compare them as well? That is what this is all about.

 

Just for general information a vehicle's speedo is allowed to read above true speed by up to 10% but not lower than true speed. Typical error from my own observations over the last eight years is that 6% is about the average over-read on speedos.

 

This weekend I noted from the GPS systems trip computer that there was a different error in odometer reading, the speedo on the vehicle in question reads 10% over so is just about on the top limit but the odometer only reads high by a fraction under 2%.

 

I thank every body for their input but would politely request that you only post the relevant info i.e. speedo over-read percentage and odometer over-read percentage and what mileage this comparison was done over. to be of any real use it needs to be over a fairly large distance, certainly several hundred miles.

 

D.

[colin]

Another way to check distances is against mile markers on motorway, you could also check speed if you where able to keep constant.

[W3526602]

Hi Colin,

 

Tell us more about these mile markers...like are they now in KM? When I was young, I passed one marker (set watch) and passed two more in 55 seconds. I'm sure that my Yugo was not capable of the suggested velocities.

 

Tyre size will affect both speed and distance readings, but the error rate should be the same on both.

 

I'm not sure about tyre pressure or loading. The rolling radius will be less, but you still have to lay the full circumference of tread onto the tarmac.

 

Yes, the M25 should be longer when travelling clockwise....same as the top of the mast on the Golden Hind travelled further than the keel.

 

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[Brambles]

davenewell@home - 2007-08-28 7:06 AM

 

Thanks Tony. As far as I'm aware the only topic of discussion on this thread is the apparent error difference between the speedo and odometer on my wife's car. There will be a slight difference between GPS measured distances and true distance covered for the reasons I gave above but unless you go round very tight corners very quickly as a matter of course then the difference will be very low. As I've said earlier this would not account for the difference between the speedo and the odometer. As they are both driven from the same signal it would seem natural, at least to me, to expect them to display the same error.

 

D.

 

Dave, by design the odometer is designed to be accurate, typically + or - 2%, the speedo 0 to + 10%. You will find the speedo reads higher than the odometer by design.

Hope that helps you in your quest.

Jon.

[Old Git]

Dave, by design the odometer is designed to be accurate, typically + or - 2%, the speedo 0 to + 10%. You will find the speedo reads higher than the odometer by design.

Hope that helps you in your quest.

Jon.

 

Why Jon? I could understand it in latter days when making a truly accurate speedo was quite difficult, but in the digital era there should be no problem at all????

[mikeb]

My Ford Focus has a 10% difference between speedo and Navman GPS whereas my Talbot Express AS Harmony only shows a 5% difference, both on the positive side which appears to be within the design requirements.

The folowing info may be of interest.

In the world of road running courses have to be measured to an accuracy of inches per mile. (in case anyone breaks a record). This is usually achieved using a 'Jones counter' which is a mechanical, gear driven odometer on the front wheel of a bycycle. The counter does not give distance, merely a number that is calibrated against a series of standards traceable back to a master. The measurer also has to factor in air temp and pressure on the day.

We have a 13.1 mile (half marathon) course that has been measured and certified using this method. We have subsequently found that my Focus odometer is virtually 100% accurate on the distance, whereas GPS gives varying results. On one occasion six runners ran the course together using 6 Garmin GPS units (Forerunner 201) to try and determine the accurate positions for mile markers. The result was 6 different (sometimes by up to 100yds) answers. So we did it by car odometer! so much for technology.

A factor that may have influenced the result was the route was very hilly with several 1 in 5 gradients.

 

Hope this wasn't too far off piste.

 

Mike B

[trooper]

Ive tested it many times on various motors and always the satnav is slow,

when aproaching a local 30 flashing sign ,30 on the speedo, o/k ,30 on satnav it flashes??

cheers

[W3526602]

Hi

 

The speedo on my wife's 1990 SAAB 900i (classic) matches exactly the speed shown on those flashing signs. So what error rate are those signs allowed?

 

Also, what error is allowed on the half second between photos as you cross the white lines in front of a zap camera. I just cannot accept that there is NO error margin, though I accept it may be small.

 

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