For once, everyone is correct. The two equations are identical, and will
indeed give slightly more accurate results if used with the "effective" tire
diameter (based on the actual tire distance a loaded tire travels in one
revolution).
It also turns out that the constants (188.5 divided by 63360, or 0.002975)
have nothing to do with tire brand. Their only purpose is to convert inches
to miles, and minutes to hours (and factor-in the constant PI to convert
diameter to circumference).
There are 63360 inches in a mile, and if you multiply the 60 minutes in an
hour times PI, you get 188.5.
My thanks to Bill for sending me a copy of his spreadsheet some time ago so
I could spend an otherwise perfectly good evening trying to figure this out
. . . :-)
Jim Hill
===================================
MJB wrote:
> > > Tire Diameter (in) x RPM x 188.5
> > > Speed (mph) = -------------------------------------------
> > > final drive ratio x gearbox ratio x 63360
And Bill Dentinger wrote:
> > 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.
And "mporter" wrote:
>
> Actually, both of these equations overstate mileage by about 2-3%,
> depending upon the tire. The most accurate equations for such . . .
> use the rolling circumference of the tire, rather than the calculated
> circumference.
|