How to Calibrate a Speedometer for Tire Size

The speedometer on your bicycle computer should be calibrated to the size of your bike's wheels. This ensures that the computer calculates the correct speed while the magnet on the wheel spins around. Without this calibration the speed displayed on your handlebars will not be correct, possibly showing a speed either remarkably slow or impossibly fast. Learn how to measure the size of your wheel and input this measurement into your cycling computer for an accurate speed reading on your next ride.

Things You'll Need

  • Measuring tape
  • Masking tape
  • Permanent Marker
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Instructions

    • 1

      Make a mark using permanent marker on the front tire of your bike where the tire meets the ground. This will allow you to observe the rotation of the tire, so as to measure effectively.

    • 2

      Stick a piece of masking tape to the floor next to the mark you made on the tire. This piece will mark your starting point, giving you somewhere from which to measure.

    • 3

      Roll the bike forward and watch the orientation of the front tire via the permanent marker marking. When the marking touches the floor again, stop the bike and place a piece of masking tape next to the mark.

    • 4

      Measure the distance between the two pieces of masking tape using a measuring tape. If possible, take your measurement in millimeters, as this is the type of measurement needed by your bike computer. If this is not possible, you will need to convert whatever measurement you take to millimeters before inputting the data into the computer.

    • 5

      Remove the computer face from the handlebar clamp. Refer to the owner's manual of your computer to locate the "wheel size" setting of your computer. Input the measurement taken previously into this dialog and save it, then reattach the computer face to the handlebars. On your next ride, you should get an accurate speed reading from your speedometer.