I just did my own spreadsheet using Mark's formula and if this is correct, I
could theoretically get 200 mph out of my Spitfire (If I was crazy enough
to drive it at full speed, that is)! :)
Joe
mporter@zianet.com wrote:
>
> 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.
|