I’ve been using the basic Garmin footpod for 3 or 4 years. I originally calibrated it on a 400 meter outdoor track while running precisely at 7:00 per mile/1600 pace. I hadn’t changed the calibration factor since.
For six months out of the last 3 or 4 years I’ve run regularly on several different treadmills and on an indoor 200 meter track (all at the same gym). When on the indoor track, the footpod is spot on for distance…as long as I stay close to a 7:00/mile pace. The watch I have paired with pod is set to beep/auto-lap every half mile. At a 7:00 pace, it beeps at the same point on the track (every two laps) without fail.
If I run the same 200 meter track at a 6:30 pace, the footpod will give a more generous result (my watch will beep/auto-lap about 20-30 meters early). It thinks I am covering more distance than what I actually am.
At a 8:00 pace, it goes the other way. It auto-laps after each 420-430 meters.
Just for fun today (and as a result of reading this thread yesterday) I recalibrated the footpod while running on the 200 meter indoor track at a 8:00 pace. Then, I did a couple of more miles on the same track, at an 8:00 pace. My watch auto-lapped at the proper place (not 420-430 meters late like it always did when I was running 8:00/mile with the 7:00/ calibration).
All of this tells me my Garmin footpod is accurate…as long as I run at the same pace I calibrated it at. Faster or slower and it drifts slightly.
It also tells me the displayed pace on the 12 or so different treadmills I run on at the same gym vary greatly…like up to a minute/mile off. Obviously, vanity leads me to use the generous treadmills whenever they are available. After all, if the guy next to me peeks at my pace, I want him to think I am crushing him.