JVN's Calculator (help)

[Kernaghan]

It totally depends on how precise you wan't to be, but treating a sprocket ratio as a gear ratio may be different due to gear reduces efficiency. Aluminum sprockets I believe have an efficiency of about 90 - 92 % if your using two sprockets. So if you feel like calculating things yourself !
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Gear reducer ratio = Gearbox sprocket# / Wheel hub sprocket#

Equation:

Output speed(RPM) =
Input speed(JVC outputRPM) / Gear reducer ratio * gear reduces efficiency(% ...0.92 or 0.90)

Once you have output speed(in RPM)

Calculate Circumference = Wheel diameter(in) * PI

Finally:

Speed(fps) = (Circumference * Output speed) / [60 (for seconds) * 12(get ft)]
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I hope this isn't too confusing