View Full Version : gear ratio calculator

18 September 2010, 0701
Just realized this was from the old site- found it again, but it's a calculator for getting at the gear ratios:

02 October 2010, 0816
I need some help- I think I may have done pretty well for a math-challenged photographer, but can someone tell me if this is right? It's a basic formula for figuring the gear ratio:
(motor RPM) x (tire circumference) x .000948 = reduction factor
(target MPH)

You plug in the loaded motor RPM, the tire circumference and the desired mph, you get the gear ratio.

For example, my motor RPM is 2500. My tire is 72.22" circumference (23" diam) and I want to go 65mph. I get a ratio of 1 : 2.63. If I'm running a 14 tooth front, I need a 37 tooth rear.

Do I got it? The figures seem to confirm on the sprocket calculator...

Here's how I got that magic number .000948. 1 inch = .0000158 miles. 1 RPM of the wheel will give you your miles per minute, so you multiply that by 60 for miles per hour. My inches, .0000158 times 60 = .000948. So that gives me the miles/hour of 1 RPM of the wheel.

wait. what?

Tony Coiro
02 October 2010, 0919
Looks good Ted, although you should probably add e^(i*pi) + 1 to the left side, it would make it slightly more accurate by accounting for tin foil helmet you'll be wearing.

02 October 2010, 0944
yeah shaddap about that, im waiting for the patent.

Here's a simpler version that figures the circumference:

(motor RPM) x (tire diameter) x 0.003 = reduction factor
(target MPH)

02 October 2010, 1014
I got 64.69339 mph when calculating with my spreadsheet using your example. I used Bidwell's book as a reference to make my spreadsheet.

This uses Rev/mile (5280/(3.14*Rear wheel Dia./12).

Rear Wheel Diameter was calculated with the help of an online calc.