To slow the speed at which a wheel is being turned by a gear reducer you will require a reducer, which is typically a smaller gear that is used to turn a larger gear at a slower speed.
Sizing a gear reducer is done by first obtaining the speed of the gear being powered by an engine. Making a wheel go slower is done by changing the gear ratio. To do this you will need to obtain the gear ratio for the size of gears for the engine
Measure the diameter of the larger gear with the tape measure. Note this measurement on your piece of paper. Measure the diameter of the smaller gear. For this example the measurement of the larger gear is eight inches and the smaller gear is four inches.
Multiply the diameter of the larger gear by the rpm of your engine and then divide by 336. This will give you the speed of the gear.
Draw a circle of the larger gear on a piece of cardboard with the caliper. Cut out this circle with the scissors. Draw and cut out a circle of the smaller gear.
Mark the center of the larger circle with the pencil. Push a nail into the cardboard on this mark.
Place the smaller circle against the larger circle on the left side. Hold the nail with your right hand and turn the smaller circle with your left.
Count the number of times it takes the smaller circle to make the larger circle turn one revolution. You will see that the smaller circle needs to turn twice to make the larger circle turn once. This is known as the gear ratio and is expressed as 2:1.
Repeat this process with different sizes for the small wheel to obtain the required gear ratio. When you have the required gear ratio the diameter of the smaller circle is the size of the gear reducer .