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Deviaton ODO meter Carpe Diem 09-26-2006
Posted by Bill on September 26, 2006, 3:02 pm
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>> I've a few years experience with precision measurement equipment. First,
>> you have to know the accuracy of the odometer used to place those
>> markers. Then, you have to eliminate any human factors such as the time
>> it takes you to move your eyes from the mile marker to your odometer
>> unless you came to a full stop at each marker and corrected for parallax.
>>
>> The diameter of your tires diminishes by about 3/4" as the tread wears
>> down and there are differences in the diameter between brands of the same
>> stated size. Compared to tire wear, inflating the tires from 36 psi to
>> 40 psi isn't going to make anywhere near the difference you will
>> experience from tread wear. An honest manufacturer would calibrate the
>> odometer to the diameter of the tire they ship the car with adjusted for
>> 50% tread wear. There would be an error from the beginning of the tire's
>> life, offset by the error at the end.
>>
> The car is new ; tires have less than 2000 miles.
> Anyway, 4 to 5% seems high to me..
> Did anyone do the same test? With what result?
The outside diameter of my tires is roughly 25" 4% of that is roughly 1".
Your speedometer should be roughly 2% off with no wear. Now, did you come
to a full stop at each marker? Does your odometer increment in tenths of a
mile? Mine does not hence I would have to see the miles digit increment
exactly as I crossed the starting marker.

I'm asking these question because I'm looking for the other 2%.




Posted by Carpe Diem on September 26, 2006, 3:32 pm
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>
>>> I've a few years experience with precision measurement equipment.
>>> First, you have to know the accuracy of the odometer used to place those
>>> markers. Then, you have to eliminate any human factors such as the time
>>> it takes you to move your eyes from the mile marker to your odometer
>>> unless you came to a full stop at each marker and corrected for
>>> parallax.
>>>
>>> The diameter of your tires diminishes by about 3/4" as the tread wears
>>> down and there are differences in the diameter between brands of the
>>> same stated size. Compared to tire wear, inflating the tires from 36
>>> psi to 40 psi isn't going to make anywhere near the difference you will
>>> experience from tread wear. An honest manufacturer would calibrate the
>>> odometer to the diameter of the tire they ship the car with adjusted for
>>> 50% tread wear. There would be an error from the beginning of the
>>> tire's life, offset by the error at the end.
>>>
>> The car is new ; tires have less than 2000 miles.
>> Anyway, 4 to 5% seems high to me..
>> Did anyone do the same test? With what result?
> The outside diameter of my tires is roughly 25" 4% of that is roughly 1".
> Your speedometer should be roughly 2% off with no wear. Now, did you come
> to a full stop at each marker? Does your odometer increment in tenths of
> a mile? Mine does not hence I would have to see the miles digit increment
> exactly as I crossed the starting marker.
>
> I'm asking these question because I'm looking for the other 2%.
>
First of all, in Belgium the odometer works with kilometers instead of
miles, but this doesn't make any difference, of course.
In fact, I did not use the odometer (which indeed doesn't have increments of
tenths of a km) but the TripA meter (which DOES have increments of tenths of
a km).
I did NOT come to a full stop at each marker ; this is a bit dangerous on a
highway, isn't it? But each time I waited until the marker passes the very
same "point" (I do not find a better word) of the car, from my point of view
(I hope my English language is good enough to exactly express what I
mean...).
And I repeat : I asked the question because I hoped that other owners had
the same experience (i.e. : approximately the same deviation).



Posted by Bill on September 26, 2006, 4:13 pm
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>
>>
>>>> I've a few years experience with precision measurement equipment.
>>>> First, you have to know the accuracy of the odometer used to place
>>>> those markers. Then, you have to eliminate any human factors such as
>>>> the time it takes you to move your eyes from the mile marker to your
>>>> odometer unless you came to a full stop at each marker and corrected
>>>> for parallax.
>>>>
>>>> The diameter of your tires diminishes by about 3/4" as the tread wears
>>>> down and there are differences in the diameter between brands of the
>>>> same stated size. Compared to tire wear, inflating the tires from 36
>>>> psi to 40 psi isn't going to make anywhere near the difference you will
>>>> experience from tread wear. An honest manufacturer would calibrate the
>>>> odometer to the diameter of the tire they ship the car with adjusted
>>>> for 50% tread wear. There would be an error from the beginning of the
>>>> tire's life, offset by the error at the end.
>>>>
>>> The car is new ; tires have less than 2000 miles.
>>> Anyway, 4 to 5% seems high to me..
>>> Did anyone do the same test? With what result?
>> The outside diameter of my tires is roughly 25" 4% of that is roughly
>> 1". Your speedometer should be roughly 2% off with no wear. Now, did you
>> come to a full stop at each marker? Does your odometer increment in
>> tenths of a mile? Mine does not hence I would have to see the miles
>> digit increment exactly as I crossed the starting marker.
>>
>> I'm asking these question because I'm looking for the other 2%.
>>
> First of all, in Belgium the odometer works with kilometers instead of
> miles, but this doesn't make any difference, of course.
> In fact, I did not use the odometer (which indeed doesn't have increments
> of tenths of a km) but the TripA meter (which DOES have increments of
> tenths of a km).
> I did NOT come to a full stop at each marker ; this is a bit dangerous on
> a highway, isn't it? But each time I waited until the marker passes the
> very same "point" (I do not find a better word) of the car, from my point
> of view (I hope my English language is good enough to exactly express what
> I mean...).
> And I repeat : I asked the question because I hoped that other owners had
> the same experience (i.e. : approximately the same deviation).
Under these circumstances, and with new tires, I'm surprised you were within
4 or 5 percent. I could perform this test using our mile markers however I
couldn't turn my head quickly and consistently from the marker to the trip
meter and at traffic speeds this alone would taint my results.

I'm thinking a much more accurate calculation could be made over 10 or 100
miles/km. I'll try this the next time I'm on the interstate highway.





Posted by DougSlug on September 26, 2006, 7:30 pm
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I'm not too surprised...odometers were not really intended to accurately
measure distance, but rather to serve as a common unit for measuring vehicle
usage. Think about a mechanical odometer--because the least significant
digit moves, it's difficult to say when you are at a specific point, so it's
tough to calibrate it. As a result, it really can't be used to make an
accurate measurement of distance, and certainly the precision of the
measurement is poor as well.

A digital odometer only implies more precision, but its precision is
actually lower than a mechanical odometer because you can't estimate the
fractions of a mile like you can with a rolling digit (this applies whether
you are talking about an integer mile or a 0.1 mile type display).

I assume, based on the number of significant figures in your measurement,
that you did this over a fairly large distance (like 1000 miles, otherwise,
the last few digits are meaningless). Are you converting from kilometers as
well? That could introduce a fixed error into the calculation depending on
how many digits you use in the calculation.

- Doug


>I noticed a deviation of the ODO meter (NOT the speed...).
> Actually, 1 mile on the ODO meter is in reality 1.0465 miles.
> I did the measurements more than once and always find the same deviation
> (between 4 and 5%).
> Is this normal? Is it because the tires have a higher pressure than what
> Toyota recommends?
>
>
>



Posted by Carpe Diem on September 26, 2006, 10:56 pm
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> A digital odometer only implies more precision, but its precision is
> actually lower than a mechanical odometer because you can't estimate the
> fractions of a mile like you can with a rolling digit (this applies
> whether you are talking about an integer mile or a 0.1 mile type display).
>
> I assume, based on the number of significant figures in your measurement,
> that you did this over a fairly large distance (like 1000 miles,
> otherwise, the last few digits are meaningless). Are you converting from
> kilometers as well? That could introduce a fixed error into the
> calculation depending on how many digits you use in the calculation.
>
The Prius has a digital odometer & "trip" meter ; the trip meter has a 0.1
km display.
Miles or kilometers : I did not convert anything. When 100 km are in fact
104 km, the 100 miles will be 104 real miles...
Measurements are done over rather short distances, but next week I have a
trip of about 550 miles (885 km) and I'll try to do a new measurement...




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