From: Tegger on
Again due to the age of the original threads, I'm going to start a new one
to try and summarize what the older threads appear to have flushed out. And
I was more wrong than right in my original assertions, apparently.


- Different tire sizes can indeed make a difference to your speedometer.

- What matters for speedometer readings is the reported number of
revolutions-per-mile, not the nominal diameter as calculated from the tire
size as written on the sidewall.

- Tire makers' reported revolutions-per-mile appear to be fairly close to
what would be experienced in the real world.

- When buying tires that are not of the size indicated on the sticker that
comes with your car, one needs to buy a tire that has the same number of
revolutions-per-mile as the original size in order to retain speedometer
accuracy.

- The reduction in actual rolling circumference (from that which is
calculated from the size that's written on the firewall) appears to be on
the order of 3%.

- Load on the tire also affects rolling circumference, but by less than one
percent up or down.

--
Tegger

From: Tegger on
Tegger <invalid(a)invalid.inv> wrote in
news:Xns9CC3551732D7Ctegger(a)208.90.168.18:

> Again due to the age of the original threads, I'm going to start a new
> one to try and summarize what the older threads appear to have flushed
> out. And I was more wrong than right in my original assertions,
> apparently.
>
>

<snip>





I'm slightly revising the previous message's points for clarity's sake:



- Different tire sizes can indeed make a difference to your speedometer,
even if they are the same nominal circumference. Those differences will be
on the order of 1% or so.

- What matters for speedometer readings is the reported number of
revolutions-per-mile, not the nominal circumference as calculated from
the tire size as written on the sidewall.

- Tire makers' reported revolutions-per-mile appear to be fairly close
to what would be experienced in the real world.

- When buying tires that are not of the model or size indicated on
the sticker that comes with your car, one needs to buy a tire that has
the same number of revolutions-per-mile as the original model and
size of tire in order to retain speedometer accuracy.

- The reduction in actual rolling circumference (i.e.: down from that
which is calculated from the size that's written on the sidewall)
appears to be on the order of 3%.

- Load on the tire also affects rolling circumference, but by less than
one percent up or down.



--
Tegger

From: Fatter Than Ever Moe on
Tegger wrote:
> Tegger <invalid(a)invalid.inv> wrote in
> news:Xns9CC3551732D7Ctegger(a)208.90.168.18:
>
>> Again due to the age of the original threads, I'm going to start a new
>> one to try and summarize what the older threads appear to have flushed
>> out. And I was more wrong than right in my original assertions,
>> apparently.
>>
>>
>
> <snip>
>
>
>
>
>
> I'm slightly revising the previous message's points for clarity's sake:
>
>
>
> - Different tire sizes can indeed make a difference to your speedometer,
> even if they are the same nominal circumference. Those differences will be
> on the order of 1% or so.
>
> - What matters for speedometer readings is the reported number of
> revolutions-per-mile, not the nominal circumference as calculated from
> the tire size as written on the sidewall.
>
> - Tire makers' reported revolutions-per-mile appear to be fairly close
> to what would be experienced in the real world.
>
> - When buying tires that are not of the model or size indicated on
> the sticker that comes with your car, one needs to buy a tire that has
> the same number of revolutions-per-mile as the original model and
> size of tire in order to retain speedometer accuracy.
>
> - The reduction in actual rolling circumference (i.e.: down from that
> which is calculated from the size that's written on the sidewall)
> appears to be on the order of 3%.
>
> - Load on the tire also affects rolling circumference, but by less than
> one percent up or down.
>
>
>

And a GPS will tell you your true actual speed and keep Johnny
Giveaticket off your butt.
From: Jeff Strickland on

"Tegger" <invalid(a)invalid.inv> wrote in message
news:Xns9CC3551732D7Ctegger(a)208.90.168.18...
> Again due to the age of the original threads, I'm going to start a new one
> to try and summarize what the older threads appear to have flushed out.
> And
> I was more wrong than right in my original assertions, apparently.
>
>
> - Different tire sizes can indeed make a difference to your speedometer.
>

Of course they do. Is the difference significant? That's the question. My
car takes a 225/55x15, but I changed it to a 225/45x17 and the speedometer
error was corrected almost entirely. My truck takes a 255/65x16, but the
previous owner had 265/75x16's installed, and the speedometer error is
amplified.

In the former (the case of my car), the affect speedometer is insignificant,
and is infact an improvement over the factory tire fitment. In the latter
(my truck), the affect on the speedo is not only significant, it is adverse
if one hopes that observiance of the indicated speed will preculde a
citation for exceeding the maximum speed allowed.





> - What matters for speedometer readings is the reported number of
> revolutions-per-mile, not the nominal diameter as calculated from the tire
> size as written on the sidewall.
>

For all practical purposes, the revolutions per mile is accurately
calculated by using the tire size numbers stamped on the sidewall of the
tire. The size will tell the diameter to within a fraction of an inch, and
the circimference can then be found, which gives the information needed to
calculate the revolutions per mile.

Granted, a low air pressure condition will affect the diameter, so any
calculation should assume properly inflated tires. Having said that, a
single low-pressure tire will not affect the speedometer -- it will affect
something, but not the speedo.



> - Tire makers' reported revolutions-per-mile appear to be fairly close to
> what would be experienced in the real world.
>

Of course they do, they calculate it from the numbers stamped on the
sidewall.



> - When buying tires that are not of the size indicated on the sticker that
> comes with your car, one needs to buy a tire that has the same number of
> revolutions-per-mile as the original size in order to retain speedometer
> accuracy.
>

That assumes the speedometer is accurate with the tires it has on it
already.

If the car takes any of several different sizes -- option packages from the
factory will offer different tires -- then any of the tires on the placard
can be used on the car. One should carefully assess putting on tires that
are not listed on the placard because the very real possibility is that the
tire selection can physically strike some portion of the body or
undercarriage.

If a car comes with a placard that lists several tire options, the largest
option will give the most accurate speedometer readings. The speedometer is
required to read a speed that is ABOVE the actual speed. You cannot get a
ticket for doing 55 in a 45 and use the defense that the speedo was only
reading 40. If the speedo says 40, you will be doing 40 or less if the tires
on the car are among the sizes listed on the placard.




From: Tegger on
"Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in
news:hdmqpo$s17$1(a)news.eternal-september.org:

>
> "Tegger" <invalid(a)invalid.inv> wrote in message
> news:Xns9CC3551732D7Ctegger(a)208.90.168.18...
>> Again due to the age of the original threads, I'm going to start a
>> new one to try and summarize what the older threads appear to have
>> flushed out. And
>> I was more wrong than right in my original assertions, apparently.
>>
>>


<selective snipping in effect>



>> - Different tire sizes can indeed make a difference to your
>> speedometer.
>>
>
> Of course they do. Is the difference significant? That's the question.




You never read my revision, evidently.



>
>
> Granted, a low air pressure condition will affect the diameter, so any
> calculation should assume properly inflated tires. Having said that, a
> single low-pressure tire will not affect the speedometer -- it will
> affect something, but not the speedo.



It /will/ affect the speedometer if it's on the "solid" side of the
diff.



>
>
>
>> - Tire makers' reported revolutions-per-mile appear to be fairly
>> close to what would be experienced in the real world.
>>
>
> Of course they do, they calculate it from the numbers stamped on the
> sidewall.



I wonder about that.

C. E. White posted a bunch of tire sizes and revs-per-mile. They were
grouped around 3%, but they were quite obviously NOT straight
calculations from the nominal circumference. I suspect (prompted by
White) that belt placement relative to tread OD has something to do with
it.



>
>
>
>> - When buying tires that are not of the size indicated on the sticker
>> that comes with your car, one needs to buy a tire that has the same
>> number of revolutions-per-mile as the original size in order to
>> retain speedometer accuracy.
>>
>
> That assumes the speedometer is accurate with the tires it has on it
> already.



The factory is likely to make the speedometer slightly "fast" for
liability and marketing reasons. Do you know just how "fast"? Nor do I.
We're all guessing as to the inherent accuracy of the speedometer and
how aftermarket-sized tires may affect that.




--
Tegger