I was thinking about this more this week. I can remember times on my rides where I was using my Garmin speed sensor, but the current speed would fluctuate in random places. A couple of stretches of road in particular. One is just wide open. The other is heavily covered with trees.

If my Edge was truly using my speed sensor during the ride, it wouldn't have those fluctuations since the sensor is active.

I'm wondering if the Edge defaults to GPS for speed unless it can't get a good signal for some reason. Meaning, I wonder if it makes a difference whether we are running the speed sensor at all when outside with clear access to the satellites.