The Speed Sensor 2 attaches to the hub of either wheel. You can set up the exact wheel size manually using the Garmin Connect app or allow the sensor to self-calibrate when connected to an Edge cycling computer or compatible Garmin device. Once set up you get accurate speed and distance data every time you start moving. The new Speed sensor records activity automatically without the need to start a ride on your GPS or mobile. This is a particularly useful feature if you\\\'re not interested in having a detailed GPS file of a regular ride such as a commute but do want the miles, time, and speeds you\\\'ve ridden to be counted from day to day. Once the sensor pairs up with an internet connected device again the simplified activities will be uploaded to the Garmin Connect app and any other 3rd party apps that are linked to your Garmin account such as Strava. The auto start activity feature can be enabled or disabled in the Garmin Connect app as required. Key Features