On Mon, 30 Sep 1996, Kevin Andrews wrote:
> In an article in our newsletter, Pat McMullen wrote about tire sizes and what
>speed you are traveling with different sizes.So he had a formula. This formula
>will not work on automatic tranies.You need 3 of these 4 things.
> 1.RPM
> 2.TD= tire diameter
> 3.MPH
> 4.DR= drive ratio(final drive)
> RPM is self explanitory,you have tire size but need to get tire
> diameter...
Kevin, I'm sure Pat's formula is good enough for rough calculations, but
there is one factor left out here. Not all tires are the same diameter,
even if they have the same size designation. For example, from the
Dunlop tire web site comes this information about diameters:
For a 175/70 R13 tire:
The D65, SP40 a/s, and SP20 all are listed as 22.7" diameter. However,
the SP4N is shown to have a 23.0" diameter. I can personally vouch for
the fact that a mounted SP4 Dunlop 175/70 13 is up to 1" larger in
diameter than a mounted Firestone 505 or Michelin MX L of the "same" size.
Back around 1974, I bought "classic" 6.70 x 15 tires from Montgomery Ward
for my 1951 Chevrolet. Their diameter was a good 2"+ SHORTER than the
tires that came off, and a like amount smaller than the NOS Atlas 6.70
x15s I later purchased for the same car. (My local gas station guy at the
time called it "short sizing"; I guess it was common then, and perhaps
it still is now.
--Andy
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* Andrew Mace e-mail: amace@unix2.nysed.gov *
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* Mrs Irrelevant: Oh, is it a jet? *
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* Triumph Herald engine with wings. *
* -- The Cut-price Airline Sketch, Monty Python's Flying Circus *
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