Is there any difference in speedometer/odometer performance between
wheels that have the same overall circumference but one has thin
sidewalls and the other has wide sidewalls? I know there is a handling
performance and comfort difference. I've always preferred the looks of
the extra wide sidewalls with small wheel hubs to the more popular huge
wheels with thin tires.

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in
news:xn0gh8siuolk2q001 (AT) news (DOT) albasani.net:



Yes, there would be a difference in speedometer and odometer calibration.

The operative wheel/tire radius is not the nominal overall radius as
measured from hub center to tire tread, but a smaller "working" radius line
that is traced from the hub center to the /road/.

Notice how the tire bulges at the bottom? That's where the working radius
line ends. This working line is much shorter than the nominal radius line,
resulting in a much smaller circumference than nominal. That's how ABS-
based low-tire-pressure systems know which tire has low pressure.

The shorter the tire sidewall (as for those larger wheels), the stiffer the
sidewall, the less the bulge at the bottom of the tire and thus the less
the reduction between nominal radius and the working radius.

--
Tegger

On Wed, 04 Nov 2009 06:27:13 -0500, badgolferman wrote:

Nope. If the overall diameter/circumference is the same, the speedo will
be on the mark.

Width does not play into it.

badgolferman wrote:
Yes, maybe, no depends on the change in radius. Tire Rack and
discounttiresdirect.com have alternate wheel/tire packages you can order
and they are supposed to fit the car and speedometer error should be
minimal. My dog loves to pee on those shiny bright new wheels.

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote


If done properly, the difference is very minor and amounts to a rounding (no
pun intended) error in the range of a few percentage points. Depending on
the goal, the difference can amount to correcting the natural error of the
speedometer to requiring entirely new gears sets to bring the operating
envelope of the engine back to a useful range.

Jeeps and trucks like to put oversize tires on that can alter the overall
gear ratio of the drive train. The cure for the problems this causes is to
replace the gear set (ring and pinion gears) with a new set of a different
ratio. Stock gears might be 3.55:1, for example, and the new tires require a
change to 4.10:1 to bring the torque and horsepower curves back to a useful
range.

Since we're talking about Toyotas, and presumably passenger cars, it would
be difficult to put tires on that require a change of gears. I wrote a
calculator that tells me the size of a given tire, and I can then plug in
other tire specs to see what the size of an alternative might be. Generally,
I use it to determine the specs that would be needed when the rims are
changed from 15 to 17, for example. As a general rule, if you have a
225/55x15 and want to see what happens with a 17" rim, you would need a
225/45. My calculator says that for every inch change in the size of the rim
you would need a 5% change in the aspect ratio -- the middle number of 225 /
55 x 15. If you wanted to make the second number larger but keep the same
rim diameter, then you'd have to go smaller on the first number.

In the example tire spec that I used, 225 is the width of the tread, 55 is
the height of the sidewall as a percentage of the tread width, and 15 is the
diameter of the rim. 225 is the width in millimeters of the cross section of
the tire (the width of the tread, more or less) and 55% of that is the
height of the sidewall.In this example, the sidewall height is 123.75mm, or
about 4.87 inches.

On Wed, 04 Nov 2009 08:06:34 -0800, Jeff Strickland wrote:


Hey! I did too!

But I lost it. It's on a disk around here...somewhere...

However, there is always one available here:

http://www.miata.net/garage/tirecalc.html

----- Original Message -----
From: "Tegger" <invalid (AT) invalid (DOT) inv>
Newsgroups: alt.autos.toyota
Sent: Wednesday, November 04, 2009 7:26 AM
Subject: Re: wheel circumference



While agree that there may be a small difference (a very small difference)
in rolling diameter for tires of the same overall diameter but with
different inside (wheel) diameters, I don't entirely buy you explaination of
why.

For sure you are right about how the ABS based low tire pressure sensors
work, but they take miles of driving to detect a very significant difference
in tire pressure. BUT.....

Modern radial tires are not like hard wheels, they are like tank treads. The
rolling diameter is mostly based on the diamter of the steel belt in the
tire as long as the tires are properly inflated. This belt has to make one
revolution per revoltion of the tire. I know this is obvious, but think
about the implication. There is very little stretch in the steel belt. Now
think about how this relates to the tank tread analogy....Even though the
tire flattens out at the road surface, every portion of the belt still
passes over the road in a almost exactly a 1 to 1 ratio. This would only be
false if the belts stretched a lot - which they don't or if they took a
really strange wavy or concave shape at the road interface, which also
doesn't happen - at least if the tire is anywhere close to properly
inflated.

Any how back to the original question:

Here is a chart for Michelin Pilot Sport Tires that relates tire size,
overall diameter, and revolutions per mile:

Revolutions
Overall Per
Tire Size Diameter Mile
245/40ZR17 24.7" 841
225/45ZR17 24.8" 837
255/35ZR18 25.2" 824
225/40ZR18 25.3" 821
265/35ZR18 25.3" 819
255/40ZR18 25.9" 770
285/35ZR18 25.9" 804
255/45ZR17 26" 798
255/45ZR17 26" 798
255/40ZR18 26" 800
285/35ZR18 26" 800
225/40ZR19 26.1" 797
265/40ZR18 26.6" 782
245/45ZR18 26.8" 780
245/45ZR18 26.8" 780
245/45ZR18 26.8" 775
255/45ZR18 26.9" 771
285/40ZR18 26.9" 771
235/50ZR18 27.2" 762

In general I don't think you can decide that tires with the same overall
diamter but different wheel sizes will have greater differences in rolling
diameters than tires of the same size but different brands or models
(construction techniques). In any case the difference is very small (1% or
less).

Ed

"C. E. White" <cewhite3remove (AT) mindspring (DOT) com> wrote in
news:IdednamjeIFc2G_XnZ2dnUVZ_gKdnZ2d (AT) earthlink (DOT) com:




I think you need to go do some actual observation and measurement. Go
outside and measure those distances on your own tires.

I'll use my own car's front tires as an example for illustration:
My tire size is 195/60-14. That gives me a nominal diameter of 23.21".

An actual (as best as I can eyeball) diametrical measurement reveals 23",
when measured across the unloaded portion of the tire from front-to-back.

That means the unloaded radius is 11.5".

If I measure from dead-center of the hub to the road (the loaded, or
"working" radius) however, I get 10.625".

That's 7/8" difference, or about 8% less than the unloaded radius.

Now...

Consider my REAR tires. The car has 61-39 front/rear weight distribution.
Same tire, same pressure, much lighter loading. The working radius here I
measure at 11.0625". The rear tires thus have a 4% larger working radius
than the fronts.

It is impossible to have a contact patch on the road unless the tire
develops a "flat spot" where the tire contacts the road. This has nothing
to do with the steel belt or anything else. Even a hard-rubber towmotor
tire has a flat-spot and a contact patch. The center of the contact patch
is the end point for that "working" radius line.

If you have bigger wheels and shorter, stiffer tire sidewalls within the
same unloaded diameter, the loaded distortion will be less, which means the
wheel/tire assembly will have a larger working circumference. How much
more? Possibly up to four or five percent, possibly as low as one or two
percent. Depends. But there WILL be a difference.




--
Tegger

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

I tried really hard to get you to see this differently. Go back and
rear my prior post. Tires are not hard wheels. The revolutions per
mile is not directly related to the loaded radius of the tire in the
manner you are suggesting. It is likely a factor, but a very minor
one....

Think! Car tires are not hard wheels like forklift tires.

How can what you are saying be true? Look at it a different
way...every time the tire goes through a revolution, there is
apporximately a one to one relationship between the tire and the road
surface (every part of the tire contact the road). One revolution of
the tire will move the car forward by the circumfrence of the tire. If
it doesn't you have to have slip (which will be true if you spin the
tires, but is a negligible factor if you are cruising straight ahead
on a level road at a moderate sped). The fact that the tire flattens
out at the road surface doesn't change this. Of course rubber is
flexible and can stretch/shrink, which is why the really important
circumfrence is the circumfrence of the steel belts inside the tire.
The treads area can stretch and flex (think about how tank treads
move) but the steel belt stretchs only by tiny amounts. Unless the
tire is so underinflated that the tire assumes a concave shape at the
road surface, the effective rolling radius is not significantly
dependednt on the distance from the road to the wheels center axis.
Like most real world things, there are other factors that come into
play, so I don't doubt that there is some effect on revolutions per
mile related to tires of the same outside diameter with different
inner diameters (i.e., wheel size), but the I don't think the loaded
radius is the prime reason.

One more thing to think about. If you are right, where does the extra
tire go? You are suggesting that one revolution of the tire will
result inthe car moving forward by a distance equal to the loaded
radius of the tire times 2 times pi. But the actual circumfrence of
the tire is the unloaded diameter of the tire times pi. Since 2 times
the loaded radius of the tire is less than the unloded diameter of the
tire times pi, this implies something is happening witht he "extra"
circumfrecne in your senario. Where is it going? You might convince me
that the rubber portion is stretching and shrinking to accomodate your
theory, but what is happenign with the steel belt in the tire?

One last analogy - think conveyor belt.....

Did you look at the chart I included with the prior note?

Ed

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

None of that matters to the ABS if all tires on the ground are the same
size, even if the tires are not the size that is specified for the car. The
ABS/Low Tire Pressure systems look at differences in the rotational speed of
one tire vs. the others. If all four tires were the same, then the speed of
the tires would also be the same and the ABS would function properly.

The tire size _can_ influence the speed display on the speedometer, but as
has been shown in several charts, if the stock size is changed for an
aftermarket size that is properly equivelent, the influence over the
speedometer is insiginficant, and if the new tire is a few percent larger,
the actual effect on the speedometer is desirable -- it makes the speedo
display the proper speed instead of display a lower speed than one is
actually travelling.

My car came with a 205/50x16, the car before it came with a 225/55x15, I put
a set of 225/45x17s on both of these cars, and all three tires are
functional equivelents for one another. (I had a car that had the 15s on it,
I put on the 17s, and a lady in a double-know pants suit and blue hair
slammed into me. I bought the car with the 16 on it, and move the 17s to
it). The affect on the speedo was that the 17's changed the error at 80mph
from almost 5mph to just under 2mph. Foremrly, the speedo would read 80 when
doing 75-ish, now it reads 80 when doing 78-ish. But since all four tires
are the same size, and functional equivelents, the ABS system still works
right.

The OP wants a larger sidewall for some reason, not a smaller one. (I don't
get the logic he's using, but it's not my decision.)

If he wants to go down an inch on the rims he's using, then he would go up
5% on the aspect ratio (the center figure in 190 / 55 x 15) to keep the same
tread width. If he wanted to keep the same rims but get a larger sidewall,
then he's have to get a thinner tire.

On second thought, I'd have to run the numbers, I'm not sure he can get more
sidewall on the same rim because if the width got less but the aspect ratio
went up, the result would be the same sidewall -- 190 x .55 and 180 x .6 are
different by 3.5mm, hardly a useful difference for any practical reason. He
would get 10mm less rubber on the ground and virtually no change in the
sidewall.