Most basic electronic bicycle speedometers allow riders to keep track of speed and distance ridden. For the device to display accurate statistics, you must program it with your wheel size. A sensor records its revolutions as you ride. Most bike computers allow you to locate your wheel size (on your tyre's sidewall), find its accompanying code in your instruction manual and input it into the computer. If you don't have an owner's manual or desire better accuracy, use the roll-out method for setting the wheel size on your bicycle speedometer.
Inflate your tires to riding pressure. Look for a recommended psi (pounds per square inch) range on the tire sidewalls. When calibrating the speedometer, you want to simulate riding conditions as best as possible to ensure accurate measurement.
Place a piece of tape or make a mark with a marker on the ground. Use a flat, smooth surface. Locate the valve stem on your front wheel directly above the starting position you marked, the 6 o'clock position.
Move the bike forward one full wheel revolution. Stop when your valve stem is back at the 6 o'clock position. As you make this revolution, put pressure on your handlebars to simulate the weight of a rider. As with ensuring proper air pressure in your tires, this will help increase the accuracy of your measurement. Mark the point on the ground where you complete one full wheel revolution.
Measure the distance between the two marks in millimetres. Multiply inches by 25.4 to generate a figure in millimetres. This number is your wheel size setting. Follow the instructions in the speedometer owner's manual to input this number into your computer. If you do not have the owner's manual, you might have to scroll through the computer's menu options to determine how to access the proper menu. The exact process for doing this varies from computer to computer.
A common road bicycle tire size is 700 X 23. If you have a 700 X 23 tire, your wheel size setting figure should be close to 2105. For 26 X 2.0 tires, a common size for mountain bikes, you wheel size setting should be close to 2066.