Nearly all treadmills have a digital readout that indicates your speed, but like any other piece of technology, errors can and will happen. If you feel like your treadmill's speed reading is either too fast or too slow, you can test it yourself manually with a few simple pieces of equipment and a little math.
Find the length of the belt. This should be listed in your owner's manual, but if it isn't listed or if you aren't looking forward to digging through a mountain of dusty boxes in the attic to find it, you can measure it yourself. Tape a piece of string to a point on the belt, then wind the belt around until it does a complete loop. Mark off and measure the string to determine your belt's length.
Mark the belt with a brightly-colored piece of chalk or piece of tape. Set the treadmill to a set speed, and after a few minutes to let it get up to the correct speed, count the number of times you see the chalk mark over a 60 second span. For example, say you set the testing speed to 2 mph, and saw the chalk mark go past 20 times.
Determine your speed by using this equation. Multiply the number of times you counted the chalk mark by the length of the belt in inches. This will determine how far the treadmill traveled in inches over the course of a minute. Take that number and multiply it by 60 to see how many inches it covers in an hour. Divide that large and unwieldy number by 12 to determine how many feet it traveled over an hour, then by 5,280 -- the number of feet in a mile -- to see how many miles you covered. For example, say that using the step earlier you counted 20 chalk marks at 2 mph, and the length of your belt was 105 inches. This comes out to 2,100 inches traveled in a minute. Multiply that by 60 and you get 126,000 inches per hour. Divide that by 12 and you get 10,500 feet per hour, then divide that number by 5,280 and you get 1.988 miles per hour, indicating that your treadmill's digital readout of 2 mph is slightly higher than its actual speed.
- Repeat the measurement three times to make sure it's accurate. If your results are the same all three times, and your treadmill is off by more than a fraction of a mph, you may want to have your treadmill serviced.
Todd Maternowski began writing in 1996 as one of the co-founders of "The Chicago Criterion." He joined the local online news revolutionaries at Pegasus News in 2006, where he continues to work to this day. He studied religion at the University of Chicago.