At that level of precision we should also consider inflation pressures and car
weight!
Overall, if you use tire revs per mile, you will find that most tires are a
bit smaller than the measurements would indicate. The 185/60-14 should be 5.9
feet in circumferance, yet 910 revs per mile implies 5.8 feet.
As for another data point, when I switched the street tires to the G-Forces on
my Panoz, the difference was 2 mph at 60 mph. While both sets of tires had the
same "size", the G-Forces were about 3 % smaller. This actually made the
speedo / odo more accurate, as previously they were reading slightly below
actual values. Perhaps this is because the street tires have bigger tread
blocks? Who knows. . .
-----Original Message-----
From: John J. Stimson-III [mailto:john@idsfa.net]
Sent: Tuesday, October 23, 2001 23:36
To: Michael R. Clements
Cc: John J. Stimson-III; ba-autox@autox.team.net
Subject: Re: Classes and course speeds
According to the specs for the AVS Intermediate on Yokohama's web
site, that tire makes 910 revolutions per mile in the 185/60-14 size
and 913 revolutions per mile in the 195/50-15 size. Which yields 0.3%
shorter gearing, not 4% longer.
Among nominal size, measured circumference, and measured distance per
revolution, the last is most accurate measure of how the tire affects
your gearing. And of course, what you measured may be true for some model
or combination of models of tires. But I don't think that it's
typical. Most of the tires I've researched (by looking up
manufacturers' specs) have all had roughly the
same value for revolutions per mile in a given size.
The 205/45-16 are bigger around, or at least cover more ground: the
Yokohama A520 in that size is listed with 896 revolutions per mile.
About 2% longer gearing.
--
john@idsfa.net John Stimson
http://www.idsfa.net/~john/ HMC Physics '94
On Tue, Oct 23, 2001 at 09:15:56PM -0700, Michael R. Clements wrote:
> Well it's true that according to the math, they are very close:
>
> 195 mm + 381 mm = 576 mm
> 356 mm + 222 mm = 578 mm
>
> Less than 1% as you say. But this is a case of theory vs. reality, as you
> suggest in your last sentence.
>
> Remember tire size numbers tend to vary slightly from one manufacturer to
> another. The numbers I have are from actual measurements of the wheel
> circumferance of these cars using the stock tires. These were 5.74 feet for
> the 15" wheels, versus 5.53 feet for the 14" wheels.
>
> So if you do the math you expect both wheels to have the same circumferance
at
> about 5.9 feet, but if you measure the actual circumferance they are smaller
> and different. And the difference of 5.53 versus 5.74 ends up being about 4
%.
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