To set the crank length for your P1 pedals on a Garmin Forerunner 920XT, you will need to have the 2.5 version of firmware. After version 2.5 Garmin released three other firmware versions: 3.3, 4.2 and 5.2. The option to set crank length in versions 4.2 and 5.2 does not appear for pedal-based power meters that are not Garmin Vector Pedals. If your crank length happens to be 172.5mm, then you are good to go. If your crank length is not 172.5mm, your power will be off by a percentage based on the difference in crank length. For any issues related to future firmware functionality, we recommend contacting Garmin Support.