Deviaton ODO meter

Would you mind telling us how you performed these measurements? I'm presuming the deviation is from a calibrated standard.

"Bill" schreef in bericht news:ObGdnbF99PPuw4TYnZ2dnUVZ snipped-for-privacy@onvoy.com...

I don't know how the situation is in the US, but where I live (in Belgium) there are milestones (is that the correct word?) along the highway (big ones each kilometer, small ones each 100 meter). So its very easy to calibrate, assuming that the milestones are placed in the right place... But as I said : I did the test on different highways, invariably with the same result.

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%.

"Bill" schreef in bericht news:jZKdnUYl1fi05ITYnZ2dnUVZ snipped-for-privacy@onvoy.com...

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.

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

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...

With a 0.1 mile accuracy on the odometer, you would only have to measure 10 miles to have 0.1% accuracy on the measurement. This would also be enough distance to over-ride any error from taking the measurement while moving and having to judge the stop and start points on the fly. If you are off by 4% after a 10 mile trip, I would agree that the odometer is off - or the mileage markers are off.

MarkB

Carpe Diem wrote:

"mark b" schreef in bericht news: snipped-for-privacy@k70g2000cwa.googlegroups.com...

I agree, and as I cannot imagine that ALL the mileage markers (along different highways) are off, I guess it's the odometer which is...

But Mark, aren't you assuming less than a 1% error in mile marker placement and ignoring tire wear and the attendant +/- 2% error? If the tires are new (2%) and the markers are off (1%) and if readings are taken on the fly, 1% after a 10 mile trip is meaningless. Tire diameter is significant to such an extent the Nav system includes a recalibration procedure to be performed in conjunction with installing new tires.

We know this much: Since you have new, OEM tires, it should be off 2%. When they are worn out, it should be off 2% in the other direction.

That will be a more useful measurement.

My point was that you had calculated the error to something like 4 decimal places, which means you are introducing more precision into the measurement than is actually there. That can be misleading, especially if rounding off is involved.

Could this also affect the MultiFunction Display of Miles Per Gallon?

I guess it does...

Good question. Tire diameter will affect a manual calculation of mpg to the same extent it effects the odometer reading. I'm thinking the Nav system calibration is necessary to keeping the positional icon properly situated on the map display between GPS readings but with some clever programming it could be used to calibrate the odometer. Maybe one of the Prius techs who visit this group knows the answer.

I put new tires on and found a difference but not significant:

30 mile - mile markers 30.1 miles - ODO

Roughly about a 3% difference.

Bob Wilson

Actually a 0.3% difference.