"Tegger" <invalid (AT) invalid (DOT) inv> wrote


That is not entirely true. For example, the Camry has several different tire
sizes that can be fitted by virtue of the different trim levels and various
option packages. The speedometer is not calibrated for each different size
of tire package, it is calibrated for the largest tire package. Smaller
tires will have greater speedometer error than the larger tires. If the base
model Camry gets a 195 / 60 x 15, the fully loaded can have a 215 / 45 x 17,
and the diameter of the tire is different by less than one-half inch. The
speedo calibration can be identical across the model line, meaning that one
could buy a base model car then get upgrade tires and wheels and still use
the same speedo.



That's certainly true. The differences you are telling us about are there,
maybe, but utterly insignificant UNLESS one tire is different than the rest.
The small difference could be enough to trigger the Low Pressure Monitor if
said monitor derives its information from the speed sensors.

"Jeff Strickland" <crwlrjeff (AT) yahoo (DOT) com> wrote in
news:hd069m$b9l$1 (AT) news (DOT) eternal-september.org:



But they're all OEM, which is my point.





--
Tegger

"Jeff Strickland" <crwlrjeff (AT) yahoo (DOT) com> wrote in
news:hd05hr$6ok$1 (AT) news (DOT) eternal-september.org:



As I said all along. The point is that it is unlikely that there will be
NO change.

Another example for illustration:

Assume a tire with 23.21" diameter (my 195/60-14). The sidewall of that
tire measures 4.6". The front tires drop about .875" at the contact
patch. That's about 19%.

Now assume a tire of the same diameter, but a hypothetical size of
195/35-18. This gives the sidewall a height of 2.7". Now imagine that
this same tire has a drop of .875" at the contact patch. That's
/one-third/ of the sidewall height of only 2.7"!

NO 35-series tire drops 7/8" at the contact patch. It cannot. Any owner
of that tire would be convinced his tire was flat! And this means the
effective rolling circumference of such a tire would be /larger/ than
the original 195/60-14, giving a slower speedometer reading..





If the wheel is turning at a speed other than what the speedometer is
expecting, the speedometer will reflect that. Period. It may not be
enough to notice in everyday driving, but it will be reflected.

ABS-based speed sensors only track DIFFERENCES between wheels, not
absolute speeds. And if a lower pressure makes a difference to the ABS,
it will also make a difference to the speedometer, provided the lower
pressure tire is on the wheel that's on the "solid" side of the diff.


--
Tegger

"Tegger" <invalid (AT) invalid (DOT) inv> wrote

Very ture, but they work by comparing the response of similar tires
and take a relatively long time to figure out that one is runnnig at a
much lower pressure than the others (miles of driving). Lots of thing
affect the number of revolutions per mile for a tire, but most are
minor. Even the speed of the car has an effect. My only point is, you
can't treat a tire like a hard rubber wheel. The loaded radius
(distance from the axle centerline to the ground) is not the most
significant factor in determining the rolling diameter of the tire.


I did not say that there was no effect, just that it is a major
effect. The ABS tire monitoring systems compare the response of the
tires. A tire with a much lower pressure than the others will on
average have a different rolling radius, but it is a very small
difference. Thise systems takes miles to figure out a tire is
underinflated.

The point was that the rolling circuimference of tires with similar
outside diamters but different rim diamters are usually very close..

Again, I agree that these ABS systems work by detecting changes in the
rolling radius of the tire, but the effects are very small and the
system take miles to figure out that the tires are different. The
rolling radius is affected by many things, temeprature, speed,
inflation pressure, even the particular road surface, but the ABS
based systems work by comparing tires to other tires on the car. They
can detect samll changes that affect one tire comapred to the others.
Therefore the small effect on the rolling radius due to a large change
in air pressure (>20% decrease) is detectable by these sysems

I am sure you are greatly over emphasizing the contribution of the
loaded radius on the rolling diamter of tires, but don't know of any
other effective arguements. Maybe an experiment would convince you. If
you have the time, measure the loaded radius, mark the tire, move the
car for 100 revolutions of the tire, and then measure the distance
moved...You will find that it moved a significantly greater distance
than 2 x pi x loaded radius x 100.

Ed

"Tegger" <invalid (AT) invalid (DOT) inv> wrote


I don't see that it matters if all tires are the same equivelent of the
original specification, or that they are all the same non-equivelent --
physical fitment issues aside.

ABS does not look at different speeds of the tires, it looks at some tires
turning and some tires not turning, it makes the not-turning tires turn. The
speedo would not care about one tire turning at a different speed, ever.

But none of this is in the realm of answering the OP's question. His
question was not framed very well, but that's another discussion all by
itself.

In the example of the 195/60x14 and the 195/35x18, the sidewall goes from
4.6 to 2.7 inches, but the tire diameter goes from 23.2 to 23.4, and the
tire circumference goes from 72.9 to 73.4, and the revolutions per mile go
from 869 to 863.

It's possible that in such a small sidewall that the pressure monitor might
not work very well, or at all, if it's based on feed from the speed sensors.
This might be an undesirable affect.

But the speedo would be unafftected for all practical purposes. Since the
new tire is miniscually larger, then the speedo would slow a tiny amount --
a current reading of 70 might be an actual speed of 66 and the new reading
of 70 might be an actual speed of 68, which is a desirable change as opposed
to an undesirable one.

"C. E. White" <cewhite3 (AT) mindspring (DOT) com> wrote in news:hd18tv$dr6$1
@news.eternal-september.org:



I must be nuts, because I actually went out and tested your theory (and
mine). I hope you do me the grace of actually reading this, because I did
perform the test instead of being Usenet-snarky and telling you to go and
do the test yourself.

After making a gauge with corrugated cardboard, I discovered that the
unloaded diameter of both front and rear tires is 23.25" dead-on.

23.25" x 3.14159 = 73.04" unloaded circumference.

Tire pressures at time of test were all the same, 31 lbs (checked hot).

On a flat, newly paved industrial parking lot, I marked the tires (and the
lot) with chalk. Leaning out the window, I then slowly rolled the car so
that the mark on the left tire described ten revolutions, coming down to
the very bottom again. That covered almost 60 feet (100 revolutions was not
practical for me.)

I did this four times each, for the front and then for the rear tires (both
sides). The results were very consistent.

The results?
Actual distance covered for the fronts: 704.5"
Actual distance covered for the rears: 708.5"

Now, how about the "loaded" radius?
For the front left, it's 11.75", which gives a circumference of 67.54"
However, that tire actually covered 70.45" per rev, not 67.54". But at the
same time it was not covering 73.04" either.

For the rear left, the loaded radius is 11.0625". This gives a
circumference of 69.51". That tire actually rolled 70.85" in the test.

Let's summarize, as percent reduction from unloaded to loaded:
Front hypothetical rolling circum based on loaded radius: 7.5% less
Front actual rolling circum based on test result: 3.55% less
Rear hypothetical rolling circum based on loaded radius: 4.8% less
Rear actual rolling circum based on test result: 3% less

Looks like the fronts split the difference between unloaded and loaded, and
the rears were affected pretty close to what I theorized.




--
Tegger