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From: badgolferman on 4 Nov 2009 06:27
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.
From: Tegger on 4 Nov 2009 07:26
"badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in
news:xn0gh8siuolk2q001(a)news.albasani.net:

> 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.



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

From: Hachiroku on 4 Nov 2009 08:30
On Wed, 04 Nov 2009 06:27:13 -0500, badgolferman wrote:

> 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.

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

Width does not play into it.
From: Fatter Than Ever Moe on 4 Nov 2009 08:36
badgolferman wrote:
> 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.

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.
From: Jeff Strickland on 4 Nov 2009 11:06

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


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.











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