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 tire, however, you also have the car's speed. In fact, this is how simple speedometers work, like those on bicycles.

- Skill level:
- Easy

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## Instructions

- 1
Look up the rpm of the wheel in question and measure its diameter (while not spinning).

- 2
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.

- 3
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.

- 4
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