Most modern stationary bikes offer a measurement of speed, even though you are technically not going anywhere. Some bikes only offer your revolutions per minute, or RPMs, which is the number of wheel rotations in one minute. Because one rotation of a real bicycle wheel would move you exactly the distance around the wheel, or the circumference, and you already know the time -- one minute --, you can calculate your miles per hour, or MPH, from RPM.
Measure in inches the diameter of the stationary bike's wheel, inclusive of the tread, if applicable. Multiply this measurement by 3.14 to calculate the circumference of the wheel. As an example, a 20-inch diameter gives you a circumference of 63 inches, rounded up.
Multiply the circumference by the RPMs. In the example, if the bike's gauge read 500 RPMs, then you would multiply 500 by 63, resulting in 31,500 inches per minute.
Multiply inches per minute by 60 to calculate inches per hour. In the example, you would have 1,890,000 inches per hour.
Divide inches per hour by 63,360 to convert to MPH. In the example, this would result in 29.8 MPH.