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29-06-2005, 12:58   #1
shamalive
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Speed of a vehicle

Hoping someone here can answer this/

How is speed mph/kph measured in a car?

On the basis of revolutions of the axel? Or wheel?
Surely not the wheel as changing wheel size would then require
instruments to be re-calibrated to get an accurate reading.
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29-06-2005, 16:03   #2
ApeXaviour
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No no no.. All modern cars have an ether-meter in them. You see ether is all around us, it allows us to measure the motion of objects with respect to the universe. If we can measure how fast the ether is moving past the car then we know how fast the car is going once the rotation/orbit of the planet/solarsystem/galaxies are taken into account.


Ok seriously I think this is more suited to the motors forum. You already know the physics of it, your question is one of hardware that a mechanic (or mechanical engineer?) would be more qualified to answer.
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29-06-2005, 16:33   #3
nesf
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Quote:
Originally Posted by ApeXaviour
Ok seriously I think this is more suited to the motors forum. You already know the physics of it, your question is one of hardware that a mechanic (or mechanical engineer?) would be more qualified to answer.
I agree.

Thread moved.

It was based on readings off the drive train at some point, I think. Whether modern cars do this or not is not something I've ever really thought about.

Hopefully one of the more car wise types in motors will be able to answer this
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29-06-2005, 16:49   #4
dudara
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Good article on this here.

It's as nesf said, basically odometers/speedometers are calibrated to the number of turns that the driveshaft makes.
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29-06-2005, 17:01   #5
Alun
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But to answer the OP's question, yes, changing wheelsizes due to temperature / incorrect inflation of tyres / aftermarket wheels or tyres of a different to size to those originally fitted will affect the accuracy of the measurement.

In addition to this there is usually a systematic error of around 10%, i.e. the speedometer will indicate a speed that is approximately 10% more than your actual speed. This can easily be verified if you have a GPS based satnav system in your car, or even a hendheld GPS.
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29-06-2005, 17:38   #6
ibanez
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Usually a speed sensor on the gearbox.Tyre pressure,Temp and Size will affect the reading but most speedos are inaccurate anyway.Does not matter if you have an analog or a digital speedo.
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29-06-2005, 22:28   #7
 
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Quote:
Originally Posted by Alun
But to answer the OP's question, yes, changing wheelsizes due to temperature / incorrect inflation of tyres / aftermarket wheels or tyres of a different to size to those originally fitted will affect the accuracy of the measurement.

In addition to this there is usually a systematic error of around 10%, i.e. the speedometer will indicate a speed that is approximately 10% more than your actual speed. This can easily be verified if you have a GPS based satnav system in your car, or even a hendheld GPS.
part 1; Correct

part 2; The error can go both ways, particularly on the mech ones. I'm sure you have seen the flaky mech one with the needle bouncing between 10and 50 when doing 30.

Electronic ones tend to be more accurate, even if biased. ones with a needle, are most likely electronic on modern/new cars. ie there is a sensor somewhere in the gearbox and the needle is driven by some form of a motor or electromagnetic servo.
Easy tell an electronic needle from a mechanical one, as the needle tends to register a +ve MPH, even when going in reverse. With a mech one, the needle/digits want to reverse.
Electronic ones have an advantage in that they can be recalibrated to work with different wheel sizes, on the same vehicle. just program in a new "fiddle factor" into the ECU.
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29-06-2005, 22:33   #8
 
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To paraphrase what's been said above "You can't trust your speedo 100%"

Given the inaccuracies that can occur which are beyond the control of the driver, how exactly do the Gardaí prosecute with any degree of justice a motorist who claims to be driving on or just below the speed limit?
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29-06-2005, 22:43   #9
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I know from my own experience that if you change the wheel size the speedometer becomes unreliable - i.e. smaller wheel = higher speed than real indicated.
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29-06-2005, 22:55   #10
 
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Quote:
Originally Posted by Zynks
I know from my own experience that if you change the wheel size the speedometer becomes unreliable - i.e. smaller wheel = higher speed than real indicated.

surly thats the other way round...smaller wheel = lower speed than real iindicated
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29-06-2005, 23:02   #11
ibanez
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Quote:
Originally Posted by stag39

surly thats the other way round...smaller wheel = lower speed than real iindicated
No he is right a smaller diameter wheel has to rotate faster than a larger one meaning for a given speed the engine has to rev higher and the cogs in the gearbox have to turn faster giving an inacurate reading at the speedo.
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29-06-2005, 23:04   #12
Alun
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Quote:
part 2; The error can go both ways, particularly on the mech ones. I'm sure you have seen the flaky mech one with the needle bouncing between 10and 50 when doing 30.
Only on old American black and white movies Mechanical speedos are a thing of the past here. And AFAIK over here there's a regulation that a speedometer can never underread, i.e. if there's an error at all it must always be positive and read more than the actual speed, so manufacturers build in a +10% error. That's what I've heard anyway, and it's been borne out in a couple of recent cars I've had comparing their speeds with that indicated on a GPS.
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29-06-2005, 23:42   #13
 
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hey, are you calling my fine chariot a "thing of the past"?

wrt to the error being -0/+10, you could be correct on that one.
however, regulations are one thing, actuality is another.
eg, I fitted larger tyres to the back/driven wheels of the truck and it still reads 60 when doing 55.

wrt to the Gardai, don't know. but the average cop about here would wait untio you were >5mph over before bothering anyone.

I notice 0.1 mile markers on the lane divider wall here, so checking the speedo is easy... if all the other car would move over. And the "diamonds" in the carpool lane are also 0.1 apart, so not only can one check one's own speed, but so can a cop without a radar, aeroplane or calibrated speedo. Cute hoors.
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30-06-2005, 23:16   #14
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I remember trying that on the M9, using the marker posts for the exits. It proved that the speed indicated by my speedo depended on what exit I was passing - an indicated 60mph was actually 52mph at one, 55mph at another and 62mph at another. Either that or the marker posts weren't at 100metre intervals. But I'd say it was a randomly adaptive speedo.
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30-06-2005, 23:57   #15
unkel
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Quote:
Originally Posted by AMurphy
wrt to the error being -0/+10, you could be correct on that one
I think so too. From all car reviews I've read where "real" speed is measured compared to speedo, the difference is usually in that range. Observations:

1. The error generally moves from 0 (speeds up to about 50km/h) up to anywhere up to 10% (topspeed)

2. Higher quality cars have less error (better quality / better calibrated equipment?)

Quote:
Originally Posted by ibanez
No he is right
Indeed. If you change you're standard tyres for bigger diameter ones and you drive at exactly the same speed as before, your speedo will show a speed that is lower

See this little calculator

It's simple really. Divide the new circumference (pi*diameter - junior cert, remember anyone ) of your new and bigger tyre compared to the standard circumference and you'll know how much your real speed is higher than it used to be

Mind it gets a bit tricky: getting bigger size wheels doesn't necessary mean bigger diameter. The tyres obviously matter too

My car standard: 16" 235/60
My car aftermarket tuned: 18" 245/45

The actual new circumference is 1.5% less

Last edited by unkel; 01-07-2005 at 00:14.
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