Rpm, or rotations per minute, measures the rotational speed of an object. If you want to convert a rotational speed to a linear speed, such as miles per hour, you need to know the diameter of the circle the object spins around. The larger the diameter, the larger the circumference, meaning the longer the distance covered. When converting, you need to make sure you convert the units properly, which is easier when using a calculator.
Calculate the Circumference
First, enter the diameter in feet and multiply it by pi, which is approximately 3.14. For example, if the diameter of a tire is two feet, then the circumference is 6.28 feet: 2 x 3.14 = 6.28 feet.
Convert the Rotational Speed to MPH
Multiply this by the rotational speed. For example, if the rotational speed is 100 rpm, enter "× 100." This gives you a value for feet traveled per hour based on the rotational speed and the circumference of the wheel.
Multiply this number by 60 to convert it from feet per minute to feet per hour.
Divide this number by 5,280, which converts feet per hour to miles per hour. Push the equal sign to see the speed in mph. In this example, when you push the equals sign, your calculator displays 7.14 miles per hour.