I wrote earlier about how the Stryd felt slower when used on the treadmill, but I finally got fed up by everyone telling me that the treadmill display on my gym is wrong and Stryd is right. So I brought out all my foot pods and tested each one multiple times against the Stryd. Before doing the tests I made sure the foot pods were calibrated correctly. The Stryd is uncalibrated and set to 100.0, while other foot pods were auto-calibrated by my Garmin watch by running outside at a flat open area at a consistent pace. (Sidenote: I've tried auto-calibrating my Stryd and it ended up with 100.0 as the calibration factor. Impressive.)
The result? The treadmill display, two MilestonePods (now called Zwift RunPod), two ANT+ foot pods (identical to the Garmin SDM4), my Garmin Forerunner 935 watch alone (using the least accurate method of pace estimation of arm swinging), my perceived exertion level, my heart rate (measured using Polar H10, the Garmin HRM-RUN, or the watch's built-in optical heartrate sensor), comparison with other runners on treadmills next to me using their own watches or foot pods, all ended up having similar speed and pace, while the Stryd is NINE PERCENT (!) slower. One thing that was surprising (or not that surprising) was that the treadmill display was accurate. Surprising because everyone kept telling me not to trust the treadmill display, not surprising because the very first time I ran on it I felt it was accurate (by feeling alone) and before I got the Stryd I've used other foot pods that also reported similar speeds as the treadmill display.
After repeating the test I decided to set the calibration factor of the Stryd to 109.0, and it started giving me the same results as the other devices. The biggest problem with this is that because the Stryd is accurate everywhere except on the treadmill, I have to manually set it to 109.0 for the treadmill and 100.0 when I'm not on the treadmill. Just setting the Stryd to provide speed and distance indoors wouldn't work, since I also run on an indoor track and Stryd is extrememly accurate on the indoor track.
I've been using the Stryd for about seven months before doing the test. Yeah, at first I didn't really care (or know) about it being so much slower. I actually thought the Stryd was more correct than the others, since it gave very consistent results (1% variance) while other foot pods could give me up to 5% variance. And I attributed the slowness to factors such as the treadmill being uneven or lack of fans leading to my body overheating and thus giving false exertion factor. But after I started using Zwift running, I would join group runs or races, it's incredibly annoying to find myself lagging behind everyone else even when I'm running really hard. I tried speeding up the treadmill to my typical outdoor speeds according to Stryd and always felt that it was way too fast for me to keep up or even run safely because the belt was going so scarily fast! In fact, I "cheated" a few times by not running on the treadmill and still have Zwift open on my phone just so I could be faster.
Oh, and also some people said that pace shouldn't matter, only power matters, I also discovered that since power is based on speed, changing the calibration factor to 109.0 also increases power by 9%. Yeah, I was running my butt off on the treadmill, and it was giving me really low power numbers.
The weirdest thing is that my Stryd is completely accurate when not on the treadmill, so I suspect it must be my treadmill running form that's affecting it somehow. Running on the treadmill is really running in place with no actual forward motion, only leg motion and the belt carrying the legs, so I don't know if my Stryd is faulty in that it doesn't sense the "simulated" forward motion, or if it's the belt being softer than ground, but none of these factors can explain why other lesser foot pods are consistent with each other. The original thought of the treadmill being inclined was wrong too, because I managed to find some with 0% incline didn't change anything. Of course, one can argue that only the Stryd is right and everyone else is wrong, but that won't explain my perceived exertion level, and how could everybody all be wrong by the same amount? I realize I didn't actually measure the distance of the belt, so... who knows?