The Garmin Forerunner 920XT precision

I was wondering about the precision of our GPS watches, so I did the following three experiments. I went to the track and did all these experiments on Lane 1. I always started on the 1 mile line (that is about nine meters behind the finish line).

Experiment 1. Walked all laps steady pace. The Garmin beeped about 20 meters after I crossed the finish line. Garmin-Connect.

Experiment 2. 100m walk/100m run intervals. Garmin beeped about 10 m after finish line. Garmin-Connect. 

Experiment 3. Did the mile riding my bicycle. I made sure nobody was around. I was surprised this is how fast elite runners run the 5000 or 10000 meters! They are fast! Garmin beeped only about five meters after I crossed the Finish Line. Garmin-Connect.

Conclusions. The faster you go, the more precision.