Mike:
The formula you have is what is commonly used but, depending on the
PRECISION you want, it may not work for all cases.
Other than the RPM and "overall" gear ratio, the critical dimension that
is moving the car forward is the distance from the centerline of the
drive axel to the ground. Some folks assume this is half the diameter
of the tire. But is it? If you have a two inch "foot-print" then the
critical dimension is 2" shorter than the calculated tire radius.
Example, if the tire diameter is 27" the radius is 13.5" but if you have
a 2" footprint then the "drive" radius is actually 11.5". To account
for this phenomena I use this formula MPH = (rpm x w) / (168 x r).
W = actual measured distance from the axel-centerline to the ground.
R = "Overall" gear ratio. (i.e., trans, and rear end multiplied
together)
It is really the same formula you used but the "168" constant takes in
consideration you are using a radius instead of a diameter. Which allows
you to measure the actual true radius when the tire is in a loaded real
race condition.
With the high pressures LSR tires require there is less of a footprint
and therefore less error from you formula. Also, as speed increases tire
growth will start to assume a true diametrical shape also reducing the
errors inherent in your formula. Where my formula may come into play
would be in the lower gears where speeds are slower and the foot print
is more pronounced. You said you were looking at shift-points vs. speed
and I thought you might be interested in another opinion. Also high
speed tire growth may be different at the bottom than at the top which
my formula would allow you to compensate for. Actually, you will be
just fine with either formula, but since this is a forum to share ideas
I just thought I would give you added information to help you with your
choices. Best of luck, -Elon
mph = rpm x tire diameter / differential gear ratio x 336
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