I’ve FOUND THE SECRET FORMULA muwuahAHAHAHAHAA!
(motor RPM) x (tire circumference-in.) x .000948 = reduction factor
Plug in the motor RPM when loaded, the tire circumference (diameter x 3.14) and the MPH you’re looking to get. That gives your reduction factor. Take your front sprocket tooth count, multiply it by that factor, you’ll get the number of teeth you need on the rear.
Wait- here’s a simpler one- just plug in the diameter, it figures the circumference for you:
Here’s a simpler version that figures the circumference:
(motor RPM) x (tire diameter) x 0.003 = reduction factor