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 Post subject: Re: Speedo Accuracy
PostPosted: Mon Oct 12, 2009 18:43 
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Mole wrote:
Speedos in modern mass-produced "type" approved cars are allowed to over read by 10% plus 2.5MPH. As has been said, they are not allowed to UNDER rear AT ALL. There's an EC Directive about it, 75/443/EC. The ship's depth sounder or aircraft altimeter analogy would therefore be that ships could only have depth sounders that recorded the water as being a bit shallower than it really was and planes could only have altimeters that recorded the ground as being a bit nearer than it really was. Under our Construction and Use Regulations, it is an offence to drive a car with a defective speedo. If you try to claim you were speeding because your speedo was defective, you just add a second offence to your speeding offence!

Heavy goods vehicles can get their tachographs re-calibrated but there's no mechanism for doing so on a modern car speedo. Quite often, the error the speedo records is non-linear anyway, depending on the speed travelled at. Also, the speedo's accuracy will vary with tyre size, pressure and level of wear.


I have a 1997 ford galaxy which i have just changed the clocks on due to the old ones being damaged the problem is the speedo i just fitted reads 45mph when i am doing 30mph & the faster i go the more inaccurate the speedo is, i have been told by my local ford garage that they can recalibrate the speedo so it is accurate but it will cost me £94, so i would assume that other manufacturers are able to do the same, even if the price is a rip off.


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 Post subject: Re: Speedo Accuracy
PostPosted: Thu Oct 15, 2009 21:19 
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g4ran wrote:



I believe that only the traffic police speedos are recalibrated & if i am correct it is done twice a year.


I can't confirm the fact ,but am certain that it's a bit more regular than that . Used to have a mate at a certain MOD establishment who had access to speedo calibration kit and he said it wasn't difficult -but no details . Know from experience that the needle can be moved ( had one speedo in pre SCP days where the needle fell off ,and I calibrated it using a stopwatch and the odometer .)
As for the Galaxy - why not borrow a sat nav and compile a graph of difference .
And to toltec - perhaps if all speedo were 100 accurate , and accidents still happened , then the truth might start to seep through the mist of confusion and intrigue spread by the SCP .

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 Post subject: Re: Speedo Accuracy
PostPosted: Fri Oct 16, 2009 22:42 
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botach wrote:
And to toltec - perhaps if all speedo were 100 accurate , and accidents still happened , then the truth might start to seep through the mist of confusion and intrigue spread by the SCP .


Good point, I was thinking of speedo calibration as helping to avoid being caught by a camera, however I take it you mean that if speed really is such a large safety issue why are they allowed to be so inaccurate? Could it be that speed really is not such an issue and the 1mph drop gives a 5% reduction in casualties statistic is complete rubbish?

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
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 Post subject: Re: Speedo Accuracy
PostPosted: Mon Nov 23, 2009 06:10 
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While the point about speedos only being allowed to over-read is true. It is also not the main issue.

On older cars especially, the speedos become less accurate over time. I understand that most 80s Ford speedos still on the road are now underreading for example. And there's always the issue of people with the wrong size tyres.

At least the modern electronic ones should retain the same level if (in)accuracy until the sensor dies, then stop working altogether, but who knows. The needles are moved by stepper motors these days, is it possible for those things to become less accurate over time?


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