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Old 11th May 2009, 00:34   #1
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Default How accurate are speedometers?

Hi,
I was using vss pulses from my alto to find distance traveled (okay... okay... please...). When I relate the number of pulses with the trip meter reading, the corresponding calculated speed is almost exactly 5 km/h less than the speedometer reading for all speeds from 20 to 60km/h (i have not tested at higher speeds). I was under the impression that the new electrically driven speedos are a little more accurate than the old cable types. Please comment.

Prasad.
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Old 11th May 2009, 03:11   #2
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Quote:
Originally Posted by 9216 View Post
....I was under the impression that the new electrically driven speedos are a little more accurate than the old cable types...
Hmm. I was under the impression that speedo "error" was actually more of a on-purpose/built-in error, for safety/liability/contingency reasons with regards to indicated speed.

cya
R

EDIT : Yup! Check this out : Speedometer - Wikipedia, the free encyclopedia

Last edited by Rehaan : 11th May 2009 at 03:13. Reason: link added.
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Old 11th May 2009, 05:13   #3
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Yes, there's error on all cars.
Even noted about a 5-7% error on Honda Civic's [very new car] digital speedo in relation to the GPS.

My wagonR shows 120 on the gps when its more than 125 on the speedo.
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Old 11th May 2009, 06:48   #4
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Most speedometers are calibrated with 4% negative error.
Speedo error is a result of a whole heap of things, it can be effected by Tyre size, amount of tread on the tyre, backlash in the speedo gear, tortional elasticity of the cable and play in the dozen linkages between the transmission and the needle on your dashboard. So to keep things safe manufacturers deliberately have 4% negative speedo error. Ie. if your speedo shows 104 your really doing 100.
Its just to make sure your not going too fast, I like to think its because of speed cameras in the west. Here 60kmph and 61kmph can mean your $110 poorer and a few speeding tickets like this and you lose your license.

I’ve got 33inch wheels on my Pajero so my speedo error went up by 10%, so now its 6% positive error.
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Old 11th May 2009, 11:23   #5
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Speedo errors can also exaggerate the all-too-important FE numbers, especially on the highway
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Old 11th May 2009, 11:36   #6
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In my palio there is no error till 60-70kmph ...... after that it has positive error :-/ ie when the speedo says 96kmph car is actually doing 100kmph, tested with GPS device, i guess positive error is due to the new upsized tyres.

Last edited by Rahulkool : 11th May 2009 at 11:41.
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Old 11th May 2009, 11:36   #7
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Thats about right , its extremely important to have this calibration error when the speed cams are out to get you. Even a 2 kmph over the speed limit is enough to get you a HUGE fine. So this does help to keep your own self in check.

Quote:
Originally Posted by SLK View Post
Yes, there's error on all cars.
Even noted about a 5-7% error on Honda Civic's [very new car] digital speedo in relation to the GPS.

My wagonR shows 120 on the gps when its more than 125 on the speedo.
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Old 11th May 2009, 11:37   #8
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Speedo's have a +0% error. The negative side is more like 5-8%. This is done to try and eliminate over speeding.
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Old 11th May 2009, 11:46   #9
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Just wanted to add that even in digitally interpreted speedos (hall-effect transducers) where wheel speed is read from the ABS sensors in the wheels, manufacturers deliberately calibrate a 4% negative error.

Last edited by Gasolinejunkie : 11th May 2009 at 11:47.
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Old 11th May 2009, 14:13   #10
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Odo errors: -ve side

FE meter errors: +ve side
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Old 20th May 2009, 23:22   #11
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Odo seems to be almost accurate. It is just that when vss pulses are converted to distance to match the odo, the calculated speed is less than the speedo indication . As such the FE should not be much in error, I believe. By the way do anyone know how many vss pulses are generated per km in an Alto? My observation is about 2545.
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Old 21st May 2009, 07:08   #12
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From what I have seen is at 104 kmph highway cruising, there is about 2-3 kmph difference when compared with the GPS.
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Old 21st May 2009, 08:10   #13
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As already stated, the speedo errors are deliberately introduced for various factors. In the case of First batch Palio's there was +ve error as the meters were not re-calibrated to Indian tyre sizes.
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Old 21st May 2009, 09:00   #14
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Quote:
Originally Posted by Rahulkool View Post
In my palio there is no error till 60-70kmph ...... after that it has positive error :-/ ie when the speedo says 96kmph car is actually doing 100kmph, tested with GPS device, i guess positive error is due to the new upsized tyres.
The Palio suffered from positive speedo error, as oppossed to negative speedo error for all other cars. This meant that even though palio was just 5% less FE than the bunch, measured FE was actually 10-15% lower.
When the fuel efficient palio NV came out, the most important mod to improve FE was change the speedometer.
Was the palio always like that? No, brazilian palio had negative speedo error, but when that came to India, they upsized the tires without bothering about the speedo.
They forgot, that in India, anything can make or break a car, and hence the straw that broke the camels back was actually the speedometer!
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Old 21st May 2009, 10:36   #15
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Quote:
Originally Posted by GTO View Post
Speedo errors can also exaggerate the all-too-important FE numbers, especially on the highway
Didn't get it? FE is based on Odo reading, isn't it?
Does the negative error in Speedo indicate a positive error in Odo? I thought Odo readings were ok, based on my observation with the milestones.
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