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Upto 10%;it will vary from vehicle to vehicle because of optional tyre sizes even on the same vehicle and with tyre wear so what could be reading 5% out this week could be reading something else completely in the future.
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From experience I would say that the odometer's accuracy is usually much better than the speedo's, though odometers do seem commonly to exaggerate by 1%-2%.
10% exaggeration for a speedo is a lot - I have found perhaps 5%-7% to be more usual, though it's often not linear.
I do like to check these things...
Edited by Manatee on 13/07/2008 at 09:43
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My last car was a primera and the speedo underread by 7-9%! My current car is a citroen c5 and the speedo only underreads by 1mph. This checked with tom tom in each car.
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I don't think many people account for this error when doing their fuel consumption calculations, i daresay we all want to think we're doing better than we sometimes are.
How many have either believed the computer readout, or assumed that the 400 miles they got from a tank may well have only been 360.
Its also in the interests of the various manufacturers that customers should think their vehicle is economical, but i'm sure thats nothing to do with it..;)
Not quite so bad with a truck, but they are calibrated when new with brand new tyres, and mine new was only 1mph out according to 'drivesmart' (gps camera detector, can't abide sat nav), but its second set of drive axle tyres have just been recut, and its showing just over 55 but doing 53.
Maximus, thats a very accurate speedo in the C5, can you use tom tom to check if your odo is as accurate?
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gordonbennet, I'll try the odo on next long journey-if I remember.
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Why are we assuming that GPS is so accurate?
Do the manufacturer's state their accuracy?
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Well I believe it, mainly because it was the same sat nav, and, although dirrerent readings they were both underreading (as they should be). Incidentally, as noted elsewhere on this forum the speed on the sat nav does decrease on a hill even though speedo stays the same-hill has to be fairly steep.
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Previous forum entries conclude that sat-nat makes no allowance for vertical movement - so if you suddenly go up a 5% incline, your sat nav knows nothing about it - it only measures "as the crow flies" so can have a different reading than the speedo which is measuring the real distance travelled
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SatNav (GPS) can give a 3 dimension fix. Maybe the mapping functions on some units don't take the changes in height into account ?
My SatNav has a display which gives Elevation (altitude), though it does need a fix from at least 3 satellites to provide this. The same satnav also has a tracking / record function that gives a good indication of distance covered.
I believe the satnav works with metric units and maybe slightly more accurate showing metric units. An imperial units display may have some conversion inaccuracies.
I'd agree with the comment about the Primera speedo undereading significantly !
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To be slightly pedantic, SteVee, four satellites are required to give a 3D 'fix' - there's a 'time' element in there too....
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>> Maximus thats a very accurate speedo in the C5 can you use tom tom to check if your odo is as accurate?
>
Sorry for late reply-been away- anyway difficult to check with Tom Tom as display only counts down miles. But, I tried and it appeared that odo was within 1/2%.>
Edited by maximus on 13/09/2008 at 19:23
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Sorry for late reply-been away- anyway difficult to check with Tom Tom as display only counts down miles. But I tried and it appeared that odo was within 1/2%.>
You remembered Max, better memory than i've ever had. Thankyou.
That C5 speedo/odo is a remarkably accurate for a car unit, i wonder if they're all like that, other makers take note, but i'm coninced that a optimistic odo makes for better fuel consumption figures for drivers to convince themselves of good economy, cynical...maybe.
I know my pick ups speedo reads 70 when according to 'drivesmart' its doing 68/9mph but i haven't got anything to check the odometer, even if the vehicle's sat nav has that facility it would take me forever to work out how to use it. It wasn't that accurate until i changed tyre size to the grey import (correct) size, before that it was reading 70 whilst actually travelling a lot slower, probably 62/3mph.
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My last car was a primera and the speedo underread by 7-9%!
There must have been something wrong.
The law specifies that the reading may be up to 10% more than the true speed (plus 4km/hour) but never less than the true speed. tinyurl.com/vuj22
I disregarded the comment by Gavin Hetherington ~ he was just being pedantic.
Edited by L'escargot on 14/07/2008 at 09:10
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>> My last car was a primera and the speedo underread by 7-9%!
There must have been something wrong. <<
I agreed with the explanation that the satnav would show low 50s when the speed showed 60.
I guess that should really be described as over-read ?
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>> My last car was a primera and the speedo underread by 7-9%!
Pardon, I did mean overread. Just got a little confused.
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GPS satellites orbit 20,200km up so a small change in height would falsify the speed reading, I think not. Height measurement is available on Differential GPS system like Trimble starting at £5000+ and require a annual licence. If the road is straight then a normal £150 to £250 type GPS will settle to a fairly accurate speed if it has been constant for 10 seconds or so. A Cars are not allowed to under-read in standard spec and yes thaey can be 10% high but normally 6 to 7%. If you have an electronic speedo and LCD oddometer you would be surprised to find that the distance is correct but the speed has been uplifted to be compliant and may be save you licence. My present car offers to display the true spped digitally and the uplitfed speed on the speedo so it is quite clear what the different is. Tyre wear accepted. Regards Peter
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Interesting thread but might I add a personal observation?
Anyone who's driving through road-works and concentrating on the relevant readings of GPS and their speedometer is not paying sufficient attention to the road ahead.
it is very easy to get seriously anal about the issue of differing speedometer readings; have seen countless amonts of bandwidth wasted on it in other forums.
I suggest that any actual difference is so small as to be irrelevant in the grand scheme of things; furthermore, car drivers travelling through a 40 limit at 35 are doing so because they don't want to get nicked. Annoying perhaps but a good enough reason to do it.
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As a design engineer, I was assured by a Garmin engineer a few years ago that speed on a GPS is not calculated by Geographical data fixes as these can have relatively large instantaneous errors, Speed is calculated by measuring Doppler shift of the Satellites fixed frequency transmissions. As such (on the garmin units) speed is pretty accurate even when selective availabilty is enabled (US introduced degraded fix accuracy). Fairly safe to assume this is also the case on other units (there are only a handful of gps chip manufacturers)
Edited by brum on 13/09/2008 at 20:59
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Brum
your info is a few years out of date. There are now many GPS chip manufacturers, the US dropped jitter (degraded fix accuracy) and most units measure ground speed by geographic points. ( with a 9 satelite fix accuracy on the ground is pretty err well accurate)
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I went on a tour of the NISSAN car plant in Sunderland on Thursday through the Heritage open Days scheme. Before a car leaves the plant the car is put on a rolling road test where there are different tests carried out, one of them being the speedometer is calibrated to read 8% higher than the actual road speed.
Hope this is of some use.
Hoseman
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My old motor the Focus used to be approx 7% out, Pug is about 5%.
CBG
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My old motor the Focus used to be approx 7% out Pug is about 5%.
At 70mph on the satnav, my Almera is out by pretty much the full 10%. I think it over-reads a little less at 30, but only a little.
As with others, it's fine on distance. I wonder if the speedo was calibrated for a larger wheel size?
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>>calibrated for a larger wheel size?
Calibrated is perhaps too strong a word for anything fitted in a car's dashboard, but, yes, car manufacturers do/did make a speedo drive pinion / speedo head combination serve for as many models within a range as possible.
Now that the pickup tends to be via the ABS sensor, there's less need for that type of necessary engineering compromise.
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