Fenix 7XSS and Stryd distance measurement issue

Have you turned off 3D distance and 3D speed in the running settings of your Garmin watch?

support.stryd.com/.../360009599873-Can-I-use-Garmin-3D-Distance-and-or-3D-Speed-

You shouldn't use automatic calibration for Stryd because it's using the builtin GPS to measure the distance. Probably Polar and Garmin measured different distances by the GPS, that's why they set up different calibration factor.

Go to a 400 meter track. Set the calibration factor to 100.0 and disable autocalibration. Now run 10 round in the right side of the first line and see what you got... it should be around 4000 meter. With this test, my Stryd measured 3890 meter, so I set the calibration factor to 102.8 and its accurate like hell since 3 years.

You should also note, that different shoes and different lace fixing can effect the measuring, so if you got new shoes and you think the measuring is wrong, you can check it with the track test.

They are both off for my running (actually I never had them turned on).

I'm not using auto-calibration, I use a static value for the calibration factor - be it right or wrong. I'm not complaining about the "real distance", my issue is that it looks like my Fenix 7X is not using that calibration factor properly (or at least how I would expect - multiply the distance you got from the footpod by the calibration factor and get the final distance). Either Fenix didn't get the right "raw" distance from Stryd or doesn't do the math right - or I don't understand the mechanism they are using for getting the result.

Polar doesn't seem to have this issue - it uses the calibration factor you provided (be it 101.3% or 100%, it doesn't matter).

Also, I used the same shoes for every run, same foot pod position.

I would guess either “smart recording” which is the default in Garmin causes some cumulative difference or there is a difference in the data from Bluetooth and ANT+. The data rate for Bluetooth is higher than ANT+. This difference is detectable when using something like Kubios to analyze HRV data in dynamic exercise (BLE data has fewer artifacts).

Its also possible that Polar and Garmin do something slightly different with summing the points to come to a total distance. 

I can’t get my 7X to detect my Stryd as a BLE foot pod and when I use too many BLE devices it was causing freezes with the 7.X firmware so I’m using all ANT+ now with BLE just for music. 

My Fenix is set for 1 sec recording so the “smartness” wouldn’t be a factor. The difference between ANT+ and Bluetooth might have a contribution - I was just wondering if other people using Stryd with a calibration factor checked the distance compared with the “raw” Stryd data you would get by syncing the footpod with the Stryd mobile app. 
For me, the Stryd data would be the “source of truth”, Polar just happened to behave as expected, using the calibration factor as expected.  

I have not done this experiment but it looks like the distance is not a consistent percentage but rather a consistent absolute distance of about 40m +-10m per activity (the maximum precision is 10m). If it were a consistent percentage then the ~12km run should have twice the variance of the ~6km runs but it does not. 

I have an alternate hypothesis that your Garmin is not processing the Stryd input for a few seconds when you start the run. Maybe it isn’t connected at first and there is a shift from GPS to Footpod when it wakes up and connects. Whatever is going on adds up to about 40m. It would be interesting if you go and run some much longer runs like 15km or half marathons and see if the difference between the Polar and Garmin result is proportional to the distance or stays at 40+-10m over a longer course. The former is a calibration thing and later is some kind of connectivity thing. 

I agree that the difference of 0.6% (between Polar and Garmin) is not that significant under 10km runs so it's hard to know unless I go to longer runs (20km) if that's a "fixed" value or a real percentage - I'll validate that over the weekend. In the meantime, I hope that the Stryd team (they have this issue somewhere on their table) will be able to investigate it as well, as they know better where they should look.

I had a longer run (a bit more than 15km) today and the behaviour I've seen with my Fenix 7X and my Stryd is consistent, the difference in the measured distance is growing in line with the run distance.

What I've seen common for the runs is the fact that Garmin measured a distance with almost 0.6% lower than the Stryd multiplied by the target calibration factor (which was 101.3% initially and in the last 3 runs it was 100%).

I need to understand more about the data protocol between the footpod and the watch before making any statements or creating hypothesis about why this is happening. In the meantime, maybe the Stryd team is able to make more sense of the data they got from me.