The COROS Bike Speed Sensor calculates your speed and distance by measuring how many times your wheel has rotated. Knowing the size of your wheel circumference, it knows how far you travel in a single wheel rotation. Multiplying the rate of wheel rotation by the wheel circumference allows speed to be measured.
When pairing the bike speed sensor to a device for the first time, the device will prompt you to enter the wheel circumference of your bike to get the most accurate speed and distance measurements.
You can adjust the wheel circumference setting at any time by going to your connected device's accessories menu, selecting the COROS Bike Speed Sensor, and entering a new wheel circumference.
On your COROS DURA or COROS watch, you can access this setting by going to System > Accessories > select the COROS Bike Speed Sensor > Wheel Circumference.
How can I know my bike's wheel circumference? This is usually printed on the sidewall of your bike tire, or listed on the wheel manufacturer's website (for example, 700x25c or 2105 mm).