If you are using a cadence sensor, you should change the Wheel size setting to help increase the accuracy of the metrics provided by the sensor. The wheel size is the circumference of your tyre in millimetres (mm). To set your wheel size, do the following:

1. From the activity start screen, move down to open the Settings screen.
2. Select Wheel size, then move right.
3. Change the size so it's correct for your wheels.

To work out the circumference of your rear wheel, you can either measure the wheel yourself using a tape measure, or look up the value in an online calculator. For example, on this site you can find the circumference using the diameter of the rim on your tyre and the thickness of the tyre: http://www.bikecalc.com/wheel_size_math
For example, if your bicycle has 27 inch rims and the tyres are 20 mm thick, of the circumference is 2105 mm. This is the value you should enter.