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 %.
-----Original Message-----
From: John J. Stimson-III [mailto:john@idsfa.net]
Sent: Tuesday, October 23, 2001 17:15
To: Michael R. Clements
Cc: ba-autox@autox.team.net
Subject: Re: Classes and course speeds
On Tue, Oct 23, 2001 at 04:39:12PM -0700, Michael R. Clements wrote:
> According to the info I have ('94 through '98), that is correct. The details
> are somewhat interesting.
>
> Mazda kept the same gear ratios in the 5-speed tranny from '94 to '98 (and
> maybe later but my data stops at '98 and I'm too lazy to check the other
> years). In '98 they "shortened" the diff ratio from 4.1 to 4.3, but they
also
> increased the wheel size by an amount that happens to be the same percentage
> (coincidence??),
The gearing got 5% shorter, and the wheel size got 7% bigger, but
those only cancel if you drive on your rims. The difference in
nominal size between a 185/60-14 tire and a 195/50-15 tire is about
0.2%.
To summarize the information that was just debated and settled, all
5-speed Miatas have the same transmission gearing. 90-93 and 99+
5-speeds had the 4.3 differential gear, and 94-97 had 4.1. The
6-speeds are a different transmission, with a different differential
gearing. The overall effect is to place 1st in the 6-speed slightly
lower than 1st in the 5-speed, and 6th in the 6-speed slightly higher
than 5th in the 5-speed.
In my 6-speed, 1st is good for 30 something (happens so fast I'm not
watching the speedo). 2nd goes to 54. 3rd goes to around 75. That's
indicated; I don't really trust speedometers for accuracy anymore.
That should translate to 59 in 2nd and 84 in 3rd for 5-speed '99+
Miatas. The 90-93 Miatas should have higher limits due to the higher
redline. 94-97 Miatas would get to 51 in 2nd and 80 in 3rd.
This is all based on my observations of my speedometer at redline, and
assuming that the contribution of the tire size to the gearing is
negligible. That's probably not quite true, as the 195/50-15 and
185/60-14 are somewhere around 1-2% smaller than the 205/45-16. This
would mean that the non-2001 Miatas would reach redline slightly
earlier in each gear than I stated above. (about 1MPH in the case of 2nd).
Put non-stock-sized tires on, and all that can go out the window.
I've gotten the impression that competition tires aren't always the
dimensions that you would expect, based on their nominal size.
--
john@idsfa.net John Stimson
http://www.idsfa.net/~john/ HMC Physics '94
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