The concept of gear ratio is based on the circumference of a circle. A circle of one inch diameter will have a circumference of 3.14159 inches. (Pi is 3.14159). That is the linear distance the circle will span in one complete revolution.

A circle with a diameter of 1.27 inches has a circumference of exactly 4 inches and a circle with a diameter of 0.635 will have a circumference of 2 inches. This means that the first circle needs one revolution to cover 4 inches while the second circle will have to rotate twice to cover the same 4 inches. Therefore their gear ratio can be described as 2:1.

Most gears are built with teeth which have an advantage of not slipping and remain synchronized with each other.

It is possible to find the gear ratio simply by counting the teeth of the gears in contact. So if one gear has 60 teeth and another 20, then the gear ratio is 3:1. Gear ratios are controlled by the number of teeth even if the diameters do not measure exactly.

A circle with a diameter of 1.27 inches has a circumference of exactly 4 inches and a circle with a diameter of 0.635 will have a circumference of 2 inches. This means that the first circle needs one revolution to cover 4 inches while the second circle will have to rotate twice to cover the same 4 inches. Therefore their gear ratio can be described as 2:1.

Most gears are built with teeth which have an advantage of not slipping and remain synchronized with each other.

It is possible to find the gear ratio simply by counting the teeth of the gears in contact. So if one gear has 60 teeth and another 20, then the gear ratio is 3:1. Gear ratios are controlled by the number of teeth even if the diameters do not measure exactly.