The diameter of a tire is the distance across the tire through the center. Since tires are circular, you can find the circumference of the tire from the diameter. The circumference represents the distance the tire travels when it makes one revolution. If you know the number of inches in a mile and the circumference, you can find the number of times the wheel turns per mile.
Measure the diameter of the tire in inches.
Multiply the diameter by 3.1415 to find the tire circumference. For example, if the tire has a 20 inch diameter, multiply 20 by 3.1415 to get 62.83 inches.
Divide 63,360 inches per mile by the tire circumference to find the revolutions per mile. Finishing the example, you would divide 63,360 by 62.83 to get 1,008.44 revolutions per mile.