Posted by *Bill* on September 26, 2006, 7:02 pm

*>> 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, 7:32 pm

*>>> 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, 8:13 pm

*>>*

*>>>> 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, 11:30 pm

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 27, 2006, 2:56 am

*> 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...

>> 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?