Most rotary devices are measured in revolutions per minute, or rpm. If someone told you your car's tire rotated at 1,000rpm, that wouldn't mean much to you; the speed of the car can't be explained simply by rpm because speed is also a factor of the circumference of the wheel. If you could calculate the speed of a point on the outer edge of the tyre, however, you also have the car's speed. In fact, this is how simple speedometers work, like those on bicycles.

Look up the rpm of the wheel in question and measure its diameter (while not spinning).

Calculate the circumference of the wheel using the following formula:

circumference = diameter x pi

Say the diameter is 24 inches and pi is a constant at 3.14. Then the circumference would be 75.4 inches.

Calculate speed by multiplying the circumference by the rpm. Say the wheels are moving at 1,000rpm.

Speed = RPM * Circumference

Speed = 1,000 revolutions/minute * 75.4 inches/revolution

Speed = 75,400 inches/minute

Note: The tyre's rpm is not necessarily the same as the rpm gauge in your car, since that's measuring engine rpm.

Convert the figure to a more understandable measurement, such as miles per hour, using the following conversions:

1 mile = 5,280 feet

1 foot = 12 inches

1 hour = 60 minutes

Speed = 75,400 inches/minute / 12 inches/foot / 5,280 feet/mile

Speed = 1.19 miles/minute

Speed = 1.19 miles/minute * 60 minutes/hour

Speed = 71.4 miles/hour