Posted by Bill on September 26, 2006, 7:02 pm
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
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
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
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
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.
Posted by Carpe Diem on September 27, 2006, 2:56 am
The Prius has a digital odometer & "trip" meter ; the trip meter has a 0.1
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...