BillDentin@aol.com writes:
> In a message dated 08/08/2002 3:45:54 PM Central Daylight Time,
> mjb@autox.team.net writes:
>
>
> > Tire Diameter (in) x RPM x 188.5
> > Speed (mph) = -------------------------------------------
> > final drive ratio x gearbox ratio x 63360
> >
> > I just have this spreadsheet I wrote up years ago where you plug in a few
> > bits of info and you get the chart of speeds in the various gears. I
> > haven't
> > actually sat down with a calculator and the equation for quite some time.
> >
>
> Mark:
>
> We have similar to the above on a spread sheet as well, for the several
> different tires sizes and drive trains we run. I am not very secure about
> matters mathematical (although I like to multiply), but I am interested. We
> normally get our tires from SASCO SPORTS, and the formula they gave us was:
>
> MPH=(((ENGINE RPMs*TIRE DIA.)*0.002975)/(DRIVE RATIO*RING&PINION RATIO))
>
> Seems to be a variation. I've not tried the two equations yet to see if they
> come up with a similar answer, but 188.5 divided by 63360 = .002975, so I
> guess both would get similar answers.
Actually, both of these equations overstate mileage by about 2-3%,
depending upon the tire. The most accurate equations for such (calculating
speeds or determining pulse rates for electronic speedometers) use the
rolling circumference of the tire, rather than the calculated
circumference. The rolling circumference is dependent upon the length of
the tire patch at normal pressure, so the larger the footprint of the tire,
usually the lower the rolling circumference.
Cheers, all.
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