Often read here that speedos tend to be 'over cautious' in their readings, does this mean that the milometer is taking is measurements from the same source?
If this is so , are some cars worse than others for this?
Is this a contributing factor to some peoples poor ecomnomy?
Does anyone know exactly how accurate a speedo must be ?
Are there servicing and warranty implications if a speedo transfers a 10% error to the milometer?
Be interested to hear your thoughts chaps!!
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AFAIK speedo has to be +10% -0% i.e. can read 10% faster than you are really going but must not read slower than you are going.
I see no reason why odometer should be any more accurate, but some say they are.
Over-reading miles would show a better fuel economy rather than pooerer. There will of course be servicing/warranty implications.
I'm not sure how much a speed/odo will vary between new and fully worm tyres.
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With regards to fuel economy. I was thinking that some manufactures had more accuracy thereby leading owners to feel that economy was worse, had in mind new accord diesel in particular?
I was also wondering whether speed had an influence on accuracy ie a 'town' car versus the same car but used predominately on the motorway,which odometer would be showing the most accurate mileage after a given number of miles recorded ?
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I'm not sure how much a speed/odo will vary between new and fully worm tyres.
For a rolling radius of 300 mm, if the tread depth wears from 8 mm when new to 2 mm then the ratio of the rolling circumference from new to worn will be 300/294 = 1.02. So the odometer/speedometer will read 2% higher when the tyres are worn ~ hoping I've got it the right way round!
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L\'escargot.
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Borrow a gps and try it out in your car this give true speed and distance travelled
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With my TOMTOM the speedo in my current model Vectra was out by about 10% at 70 but almost spot on at 30.
The mileage was spot on over several distances - eg a set journey. This was the same when I changed cars from an Avensis. The miles to work places were exactly the same. So it would appear that odometers are accurate.
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>the speedo in my current model Vectra was out by about 10% at 70 but almost spot on at 30.<
To answer the earlier question, you must do the same journey at 30 and at 60, to find whether the distance measured is different. That possibility seems so silly that I think it can be discounted. As an odometer has only to count wheel rotations, while a speedo must include time in a calculation, there is no reason why one cannot be accurate while the other isn't.
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The mileage was spot on over several distances - eg a set journey. This was the same when I changed cars from an Avensis. The miles to work places were exactly the same. So it would appear that odometers are accurate.
Or the two cars had similar errors!
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L\'escargot.
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I do not have the luxury of a sat nav to check how accurate my speedo is. Recently several speed warning signs have been installed and I believe must be set very accurately to 30 and 40 mph. If I drive through at 1 mph over my speedo reading then the signs will flash. It has now become part of my drive to ensure I do not trigger any of these signs, as next time it might be a speed camera.
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Don't be so sure. There were some put up around here in a newly lowered 60 limit. They went off at 56 and above.
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Adam
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Checked against a GPS and an Origin Blue my 307 speedo over reads by 3mph right across the range.
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Speedos are just not accurate. Period.
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I am the only Pole over here.
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Checked against a GPS and an Origin Blue my 307 speedo over reads by 3mph right across the range.
A new Ford KA is also 3mph optimistic across the legal range on the analogue display.
The digital display available in "test mode" is accurate on new tyres.
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Does anyone know exactly how accurate a speedo must be ?
From: www.publications.parliament.uk/pa/ld200001/ldhansr...1
2w01.htm
Speedometer Accuracy
Lord Allen of Abbeydale asked Her Majesty's Government:
Whether, in the light of the increasing importance of speed limits, they have any plans to make it easier for the private motorist to have his speedometer tested for accuracy.[HL839]
Lord Whitty:
The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.
A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests.
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The speed measurement involves turning a rotating movement into a static needle position. In the past, at least, this was done by attaching a magnet to the end of the speedo cable and a metal plate to the speedo needle (all within the works, of course). Induction would then try to move the needle against a spring, so you'd get a reading. This would clearly be an approximation of speed that might vary as temperature, etc changed. This may have improved and been replaced with digital readings, but they still need to move a needle, which will add inaccuracies. (I exclude digital displays)
In the mileometer, cogs turn the rotation of the cable into smaller rotation in the indicator wheels. Privided the gearing is worked out correctly, this can be perfectly accurate (tyre wear excluded).
V
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Vin, in respect of digital displays, in the dashboard diagnostics on a new Fiesta, the digital speedo display reads higher than the actual speedo. Is the diagnostic speedo reading an accurate representation of the vehicle's speed on all Ford models, therefore?
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Mike Farrow
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The digital reading in mine fluctuates rapidly - probably because it's very very accurate. A quick glance to the sat nav showed that it was bob on.
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Adam
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