Treadmills have an advantage over running outdoors in that most models include a digital readout showing the belt's speed, but to confirm the speed is accurately measured requires a testing process. Though treadmills can be set to a vareity of speeds, typically from 0.5 mph to 12 mph, you need to perform a test for only one specific speed. If the digital speed reading is accurate at that setting, it should be accurate at others.
Place a strip of colored tape on the treadmill belt, running perpendicular to the motion of the belt. The tape should be thin enough to fit beneath the treadmill housing as the belt rotates.
Set the treadmill for a specific speed, preferably one of your usual running or walking rates.
Wait one minute for the treadmill to reach the set speed.
Start the stopwatch as the tape strip passes under the front edge of the treadmill housing.
Count how many times the tape passes under the front edge in one minute. Record the figure on paper.
Repeat the measurement three times. The results should be the same each time. If not, your treadmill may require maintenance.
Look up the length of the treadmill belt in your treadmill owner's manual.
Multiply the belt length by the number of revolutions you counted. This is the distance the belt traveled in one minute.
Multiply the result from step 8 by 60. For example, if the belt traveled 100 feet per minute, then the result would be 6,000. This is the number of feet the belt travels per hour.
Divide the result of step 9 by 5,280, or the number of feet in a mile. The result is the speed of the treadmill in miles per hour. A belt that traveled 6,000 feet in one hour would be moving at 1.14 miles per hour.
Tips
You can use this process to check the accuracy of the treadmill's readout by comparing the result to the speed listed on the treadmill. If it's off, you can have a service technician calibrate the speedometer or work out your distance and speed manually with the accurate measurement you calculated.