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Old 29th June 2011, 22:28   #1
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Default Odometer Inaccuracies

Whenever I go on long trips, I have this habit of noting down the tripmeter readings and time whenever a major town/toll naka comes. Over a period when i compared these reading they were not always equal for the same segments. Then i realised they might not all be talking the truth. With the advent of Google earth/maps, I tried to find the actual distance between 2 places and checked out the variances with various cars (two of them).

Assuming that the distance as measured by Google Earth is 100% correct, my wife's i10 shows 97km on the odo when the actual distance is 100 kms. And for my dad's Punto, it shows 104.1 km for the same distance. Both the cars were on stock tyres.

Am wondering if these 3-4% variances are 'normal', and whether anyone has tried to find out the error in their car.
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Old 29th June 2011, 22:35   #2
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Default Re: Odometer Inaccuracies

^^When you drive for a few hundred KM at a stretch, you can not drive in a perfect straight line all the time. As you move to the left, then right, centre and then left again, these will add up to a couple or three KM at the end of the trip.

Haven't you noticed the small difference in the KM printed in train tickets if you travel up and down to the same city from yours? Even when the up - down tracks are almost parallel?
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Old 29th June 2011, 22:42   #3
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Default Re: Odometer Inaccuracies

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Originally Posted by Gansan View Post
^^When you drive for a few hundred KM at a stretch, you can not drive in a perfect straight line all the time. As you move to the left, then right, centre and then left again, these will add up to a couple or three KM at the end of the trip.

Haven't you noticed the small difference in the KM printed in train tickets if you travel up and down to the same city from yours? Even when the up - down tracks are almost parallel?
Thanks Gansan for the quick response. I have noticed that. But it doesn't add in kms, but meters. During my recent trip, the difference between the forward and return stretch (distance 194 kms) was 0.3 kms. So the odometers are very consistent within themselves.
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Old 29th June 2011, 23:01   #4
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Default Re: Odometer Inaccuracies

Quote:
Originally Posted by Minardi View Post
Assuming that the distance as measured by Google Earth is 100% correct, my wife's i10 shows 97km on the odo when the actual distance is 100 kms. And for my dad's Punto, it shows 104.1 km for the same distance.

Am wondering if these 3-4% variances are 'normal', and whether anyone has tried to find out the error in their car.
Does google maps count the road gradient? If not, they will be showing less the distance always.

Also, regarding odo inaccuracies, YES, most cars speedos are inaccurate by 3-4%. And most times, it is more.
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Old 30th June 2011, 00:45   #5
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Default Re: Odometer Inaccuracies

Then if you've upsized your wheels it will affect accuracy of the speedometer and odomoeter as well, compared to a static map.

--Ragul
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Old 30th June 2011, 00:51   #6
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Default Re: Odometer Inaccuracies

Quote:
Originally Posted by Gansan View Post
^^When you drive for a few hundred KM at a stretch, you can not drive in a perfect straight line all the time. As you move to the left, then right, centre and then left again, these will add up to a couple or three KM at the end of the trip.

Haven't you noticed the small difference in the KM printed in train tickets if you travel up and down to the same city from yours? Even when the up - down tracks are almost parallel?
totally agree with you on this one.
On my daily commute,on the same route.
Sometimes odo shows 109.xx,sometimes 110.xxx and varies till 111.xx
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Old 30th June 2011, 01:10   #7
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Default Re: Odometer Inaccuracies

Quote:
Originally Posted by Gansan View Post
^^When you drive for a few hundred KM at a stretch, you can not drive in a perfect straight line all the time. As you move to the left, then right, centre and then left again, these will add up to a couple or three KM at the end of the trip.

Haven't you noticed the small difference in the KM printed in train tickets if you travel up and down to the same city from yours? Even when the up - down tracks are almost parallel?
Unfortunately for most highway driving this explanation doesn't work.

e.g. if you travel 1m sideways while going 10m forward, total distance travelled diagonally to only 10.05m (square root of 101) - and 1m sideway per 10m travelled is a very large number for lane change of any reasonable speed. For similar reasons, unless the road had very steep gradients throughout, google map will give a fairly accurate estimate of distance.

Though zig-zag travel (unavoidable in practice, there is no perfect straight line travel) will always be longer than straight line and google will always be shorter, so overall google will always underestimate.


A more likely explanation is tread wear. for example - for a 175/65R14 tyre the diameter of the tyre is roughly 058.3cm. For tread of a new tyre vs. old tyre, about 5mm variation is easily possible - that'll be 10mm in diameter or roughly 2% of the diameter (leading to 2% difference in circumference and hence tyre travel)
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Old 30th June 2011, 08:54   #8
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Default Re: Odometer Inaccuracies

There is speedo-odo error built into the instruments, this is a legislative requirement. The error must be positive, there is a formula regarding the same. I forgot, will post it later.

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Old 30th June 2011, 11:04   #9
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Default Re: Odometer Inaccuracies

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There is speedo-odo error built into the instruments, this is a legislative requirement. The error must be positive,
This is very interesting. Any idea why there is a legistative requirement for built-in error?
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Old 30th June 2011, 11:08   #10
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Default Re: Odometer Inaccuracies

The CMVR rules say that the display should always show a value higher than the actual and not less than the actual. I feel it is more of a psychological factor rather than anything else.

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Old 30th June 2011, 11:13   #11
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Default Re: Odometer Inaccuracies

Regarding rules of error on odo/speedometers, found this information in wikipedia.

Speedometer - Wikipedia, the free encyclopedia
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Old 30th June 2011, 11:14   #12
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Default Re: Odometer Inaccuracies

Speedo/Odo can have up-to 5% error (Not built into them by design but something to do with manufacturing i believe). On the same car/bike though you might be very close but on different cars the difference may be upto 5%.

Like for example my distance to office is 16Km, on the same car it might be few 100Mts here and there, but on a different car it might change to 17.xx or 15.xx.
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Old 30th June 2011, 11:23   #13
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Default Re: Odometer Inaccuracies

Quote:
Originally Posted by SPIKE ARRESTOR View Post
The CMVR rules say that the display should always show a value higher than the actual and not less than the actual. I feel it is more of a psychological factor rather than anything else.

Spike

Well no display can show the exact value, so perhaps govt. decided to err in the interest of safety rather than the other way round - would work for speedo anyway but what is the advantage for odo is not clear.

By the way, I read some time ago on the internet - on a test in about two dozen cars (in US) almost every car had a speedo that showed more speed than the real speed. And the fancy Germans were the biggest culprits (though the max error was about 2% only)
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Old 30th June 2011, 12:02   #14
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Default Re: Odometer Inaccuracies

Quote:
Originally Posted by mayankjha1806 View Post
Speedo/Odo can have up-to 5% error (Not built into them by design but something to do with manufacturing i believe). On the same car/bike though you might be very close but on different cars the difference may be upto 5%.
It is usually not defined by a percentage, there is a formula related with the vehicle speed.

Quote:
Originally Posted by vina View Post
Well no display can show the exact value, so perhaps govt. decided to err in the interest of safety rather than the other way round - would work for speedo anyway but what is the advantage for odo is not clear.
Agree! Regarding odo error, I feel manufacturers use it to their advantage for showing- so many KMPL's. Although, very small values but.

Spike

Last edited by SPIKE ARRESTOR : 30th June 2011 at 12:11.
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Old 30th June 2011, 14:28   #15
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Default Re: Odometer Inaccuracies

Quote:
Originally Posted by SPIKE ARRESTOR View Post
Agree! Regarding odo error, I feel manufacturers use it to their advantage for showing- so many KMPL's. Although, very small values but.
Or is it because of safety issues?. ie, a person driving a car with negative speedo error will be traveling at a higher speed than the specified speed limits. This is particularly of concern in extreme situations, like driving in snow etc..
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