Excellent Gear calculator

[futuremach1]

Here's the equation for finding what your car's MPH will be with any given RPM, given gear ratio, tranny ratio, and tire height:

(RPM x Tire Height) / (Rear Gear ratio x Tranny gear ratio x 336) = MPH

And it's pretty simple to find what your RPM will be at any give MPH also by solving for the RPM:

(MPH x Rear gear ratio x Tranny gear ratio x 336) / Tire Height = RPM


Example: a car at 75mph with a 4.10 gear and a .62 overdrive in 5th gear and 26" tires=

(75 x 4.10 x .62 x 336) / 26 = 2463 RPM

you can do it yourself using this

if you dont know your tire size, take the first number say 275 multiply it by .03936 (that will convert it to mm) multiply your answer by the width percentage of your tire (.35,.40,.45, whatever you run) then multiply that by 2, then add your wheel size (17,18,19, etc....)
example i run a 275-40-17
275x.03936=10.824
10.824x.40=4.3296
4.3296x2=8.6592
8.6592+17=25.6592