The most accurate way to test the belt speed is to use a digital rotary meter. This will translate the number of times the disc rotates and tell you within thousandths of a MPH. Since these cost about $450 US here's another easy way to do it....count the times the belt travels around in 1 minute.
On a Weslo Cadence 850 treadmill, a 104" belt is standard equipment. Due to this, the belt should travel appx. 25.5 times around in one minute. For every belt lap that the count is higher or lower, the speed is off approximately 0.1 MPH. Find a bright colored piece of tape and mark the belt, get the treadmill up to the speed of 2.5 MPH and start counting, using your timer to determine the minute.
Here's the math...Multiply the feet in a mile by the desired testing speed, then multiply the number of inches in a foot. Divide this number by the number of minutes in an hour, then divide this number by the length of the belt.
Definition of numbers in equation below: In the equation 5280 is the number of feet in a mile. 2.5 is the testing speed. 12 is the number of inches in a foot. 60 is the number of minutes in an hour. 104 is the length of this equation’s test belt in inches.
Equation 5280 x 2.5= 13,200 x 12= 158,400 / 60= 2,640 / 104= 25.38