From Garmins support pages:

- Precedence of wheel speed sensor over GPS: https://support.garmin.com/support/s...000000000000**

- Unit is continuously updating the wheel size calculation using GPS (see post#3): https://forums.garmin.com/showthread...-size-question


I can tell when the battery has died in my Edge 705, because when it falls back to using only GPS, there is a noticeable lag when abruptly changing speed.