If we are going to continue this anal discussion, the
rolling circumference of the tire with the car's
weight applied is different than the circumference
measured on your spare tire off of the car.
Better still just top end your car's speed thru your
local community's speed trap. Email us a copy of the
citation and we will all know who's is fastest.
John Holliday
--- Dan Dwelley <maine2me@yahoo.com> wrote:
> > Mark a spot on one rear tire and roll the car so
> the
> > mark touches the
> > pavement. Mark the pavement at this point. Roll
> the
> > car forward with you in
> > it (for highest accuracy) until the mark on the
> tire
> > is again on the ground.
> > Mark this spot and measure between them. This is
> > the distance the car moves
> > forward for each rotation of the axle, call it
> "C".
> >
>
> "C" for Circumference? I've got a better idea...just
> measure the circumference of your spare tire (if
> it's
> the same size as you have on your car). I bet you'll
> get the same measurement with less effort! :o)
>
> Dan Dwelley
> no wiz but I can determine the circumference of a
> circle and apply it to linear measurement. :o)))
>
>
> --- Glen Byrns <grbyrns@ucdavis.edu> wrote:
> > Here's how I figured my theoretical speed at
> various
> > rpm's:
> >
> > Mark a spot on one rear tire and roll the car so
> the
> > mark touches the
> > pavement. Mark the pavement at this point. Roll
> the
> > car forward with you in
> > it (for highest accuracy) until the mark on the
> tire
> > is again on the ground.
> > Mark this spot and measure between them. This is
> > the distance the car moves
> > forward for each rotation of the axle, call it
> "C".
> > In my case, C=69 5/8" =
> > 69.625".
> >
> > With a 4.2 rear end, the drive shaft will turn 4.2
> > times to turn the rear
> > axle one revolution. Assuming a fourth gear ratio
> > of 1:1, then at 6K rpm,
> > 6000/4.2 = turns of rear tire per minute =
> 1428.72.
> > Multiply by 60 to get
> > turns per hour, = 60 x 1428.720 = 85732.2
> > This number times the number of inches forward for
> > each turn you figured out
> > earlier(C)tells you how many inches forward for
> each
> > hour at 6k rpm in
> > fourth. Divide by the number of inches in a
> > mile(63360) and you should get
> > miles per hour.
> >
> > {(rpm/rear end ratio)x 60 x C }/ 63360
> >
> > So for a 4.2 rear end, 69.625 inches per turn of
> > tire, fourth gear:
> > (6000/4.2) x 60 x 69.625 / 63360 = 94.18 mph, at
> 5k
> > rpm = 78.49mph
> >
> >
> > I'm sure someone with a better memory of math will
> > correct me, but luckily I
> > play with DNA for a living and don't do gobs of
> > math.
> >
> >
> >
> > > I havn't seen anybody talk about tire size which
> > can have a very difinite
> > >effect on speedo error and correct top speed.
> Look
> > at any tire chart and
> > >you will see a big difference in tire
> circumference
> > with the tires sizes
> > >very close together if not the same size. Don't
> > forget to take this into
> > >consideration. Does anyone one out there have
> the
> > formula for determining
> > >top speed using tire circumference, axel ratios,
> > and rpm's?
> > >
> > >
> > >BOB
> > >65 Midget vintage racer
> >
>
>________________________________________________________________________
> > >
> > >
> > Regards,
> > Glen Byrns
> > '59 Bugeye
> >
>
>
> =====
> Dan Dwelley
> 77 Midget
> Alexandria, Va.
>
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