basically you calculate a gear ratio by dividing your driven sprocket or pulley size by the drive pulley or sprocket.
(((the driven sprocket or pulley is the sprocket/pulley that is not connected to your power source, either on a jackshaft, axle, or input to transmission, the drive is the one that is directly on the engine shaft, or other source of power)))
So for example, you have a 12tooth engine sprocket and 72tooth sprocket on your jackshaft, 72/12= 6 thats a 6:1 ratio meaning your engine will spin 6 times for every one time your jackshaft spins. Say you have a 10" pulley on your jackshaft, that runs to a 30" pulley on your axle, 30/10=3 thats a 3:1 ratio. Now to figure out what the overall ratio is, you simply multiply, 3:1x6:1= 9:1 ratio from engine to axle.
A lower gear ratio will give you more torque, but less speed, where as a higher gear ratio is the exact opposite. A 9:1 ratio will give you much higher torque, pulling power, and takeoff but a much lower top speed.
Get out a calculator and play with your ratios, with such a small engine 35p chain all around would be fine.
