tire size and speedometer accuracy

When I was hot rodding in the late '60s, I spent a lot of time researching
how tire size and different differential gear ratios affected speedometer
accuracy. I bought a lot of different speedo drive gears for the tranny
(this was on a GM TH400) to compensate for the changes I made. All this
just to keep my speedo as close accurate as possible as I didn't want to
speed and collect tickets. Now someone tells me that after putting in
larger diameter tires on his car (Honda Element), the tire dealer said that
his speedo would still be accurate. I find this hard to believe. Are
today's electronics intelligent enough to automatically adjust for different
diameter tires? I would think at least something needs to be manually
re-calibrated? This is a good group to get information even though this is
not a Toyota specific issues. Thank you.

"Dennis Leong" <spgdxl@berkeley.edu> wrote in message
news:eggn1l$16rd$1@agate.berkeley.edu...
> When I was hot rodding in the late '60s, I spent a lot of time researching
> how tire size and different differential gear ratios affected speedometer
> accuracy. I bought a lot of different speedo drive gears for the tranny
> (this was on a GM TH400) to compensate for the changes I made. All this
> just to keep my speedo as close accurate as possible as I didn't want to
> speed and collect tickets. Now someone tells me that after putting in
> larger diameter tires on his car (Honda Element), the tire dealer said
> that his speedo would still be accurate. I find this hard to believe.
> Are today's electronics intelligent enough to automatically adjust for
> different diameter tires? I would think at least something needs to be
> manually re-calibrated? This is a good group to get information even
> though this is not a Toyota specific issues. Thank you.

If the circumference of the replacement tire's tread is the same as the
circumference of the original tires, then the speedometer and odometer will
still be accurate. 2 tires can have different diameters and have the same
circumference if the aspect ratio is proportionally different. A tire's
diameter is measured from the center of the "hole" to the bead, not the
tread.
--

Ray O
(correct punctuation to reply)

I'm with you on this one. I can't see how the car would know the tires are a
different size, and alter the speedo calibration by itself.

If the original size and the new size are compliments of each other, then it
is easily possible that the speedo is not affected enough to warrant
changing gears.

I don't know the tires that the Element comes with, but assuming a size,
there are other sizes that offer more tread contact area, but are the same
diameter -- within a half-inch -- which would not adversely alter the speedo
accuracy (and the new tires would be larger, making the speedo a bit slower
and probably improve its accuracy anyhow).

For example, a 235/75x15 is 28.89 inches in diameter, wheras a 255/70x15 is
29.05 inches. The difference in DIAMETER is less than 1/4 inch. But, the
contact patch would be wider by 20 mm, or about 3/4 inch. More on the
ground, but hardly any change in speedo accuracy, and any change would
probably make it more accurate than it already is.



"Dennis Leong" <spgdxl@berkeley.edu> wrote in message
news:eggn1l$16rd$1@agate.berkeley.edu...
> When I was hot rodding in the late '60s, I spent a lot of time researching
> how tire size and different differential gear ratios affected speedometer
> accuracy. I bought a lot of different speedo drive gears for the tranny
> (this was on a GM TH400) to compensate for the changes I made. All this
> just to keep my speedo as close accurate as possible as I didn't want to
> speed and collect tickets. Now someone tells me that after putting in
> larger diameter tires on his car (Honda Element), the tire dealer said
> that his speedo would still be accurate. I find this hard to believe.
> Are today's electronics intelligent enough to automatically adjust for
> different diameter tires? I would think at least something needs to be
> manually re-calibrated? This is a good group to get information even
> though this is not a Toyota specific issues. Thank you.
>

What I can see on the Element are significantly larger (diameter) tires than
original. So I would say that his speedometer accuracy is off but the owner
really is too dense to know. Thank you.

"Jeff Strickland" <crwlr@yahoo.com> wrote in message
news:VKydneAJPrz7qKzYnZ2dnUVZ_rmdnZ2d@ez2.net...
> I'm with you on this one. I can't see how the car would know the tires are
> a different size, and alter the speedo calibration by itself.
>
> If the original size and the new size are compliments of each other, then
> it is easily possible that the speedo is not affected enough to warrant
> changing gears.
>
> I don't know the tires that the Element comes with, but assuming a size,
> there are other sizes that offer more tread contact area, but are the same
> diameter -- within a half-inch -- which would not adversely alter the
> speedo accuracy (and the new tires would be larger, making the speedo a
> bit slower and probably improve its accuracy anyhow).
>
> For example, a 235/75x15 is 28.89 inches in diameter, wheras a 255/70x15
> is 29.05 inches. The difference in DIAMETER is less than 1/4 inch. But,
> the contact patch would be wider by 20 mm, or about 3/4 inch. More on the
> ground, but hardly any change in speedo accuracy, and any change would
> probably make it more accurate than it already is.
>
>
>
> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
> news:eggn1l$16rd$1@agate.berkeley.edu...
>> When I was hot rodding in the late '60s, I spent a lot of time
>> researching how tire size and different differential gear ratios affected
>> speedometer accuracy. I bought a lot of different speedo drive gears for
>> the tranny (this was on a GM TH400) to compensate for the changes I made.
>> All this just to keep my speedo as close accurate as possible as I didn't
>> want to speed and collect tickets. Now someone tells me that after
>> putting in larger diameter tires on his car (Honda Element), the tire
>> dealer said that his speedo would still be accurate. I find this hard to
>> believe. Are today's electronics intelligent enough to automatically
>> adjust for different diameter tires? I would think at least something
>> needs to be manually re-calibrated? This is a good group to get
>> information even though this is not a Toyota specific issues. Thank you.
>>
>