Convert RPM to MPH
Do your bike workout and write down the rpm number or numbers that you’re interested in and want to convert to mph, such as your top speed or your speeds at selected intervals.
Measure the diameter of the bike’s wheel after your workout, using a tape measure or a similar device.
Multiply the rpm number by 60 to convert revolutions per minute to revolutions per hour. Multiply the result by the wheel’s diameter, then multiply the new result by pi, or 3.1416.
Divide the result from the previous step by 63,360, which represents the number of inches in a mile. For example, if your speed reaches 800 rpm on a bike with a 20-inch-diameter wheel, multiply 60 x 800 x 20 x 3.1416, for a total of 3,015,936. Dividing the result by 63,360 yields a speed of 47.6 mph.
Convert Miles to MPH
Reset the mile counter to zero, start your ride and note the time you begin, if the bike doesn’t display a running time.
Record the time you finish your workout and the number of miles displayed on the bike’s counter. Convert your workout time to a decimal. For example, convert 17 minutes, 20 seconds to 17.3 minutes.
Divide the number of miles you pedaled by the total time of your workout, then multiply the result by 60 to determine your average miles per hour. For example, if you pedal 13.2 miles in 17.3 minutes, 13.2 divided by 17.3 equals 0.763. Multiplying the result by 60 results in an average speed of 45.78 mph.