formula for calculating MPH

[steemin]

I ran across a formula that allows one to calculate MPH @ different rpm's with different transmission and rear end ratio's.
This formula would be helpful in selecting the correct rear end ratio
for your car.



MPH = (rpm X tire diameter) divided by ( *gear ratio X 336)
*gear ratio meaning: (rear end ratio X transmission ratio)

Does the math work? Is this a valid formula?
( I realize that top end is limited by h.p. and aerodynamics)
Scott