So I’ve been using the Treadmill activity to log my treadmill walks the past few days and it always logs the distance very inaccurately, like less than 50% of the distance I actually walked according to the treadmill. I usually do around 1 mile so I have to edit the distance of the activity in the app anyway, AND change the activity from a “running” to a “walking” one. After experimenting with it a bit, I would not recommend using “Treadmill” for walks because the calibration does not seem to be getting any better. It’s definitely meant to track running distance.

And no, fortunately my “Treadmill” runs track the distance pretty closely, usually within .2 miles of the distance that appears on the treadmill, so I haven’t run 🥁 into the problem you had. But I did find this info on the Garmin support site:

The calibration value input cannot be less than half or more than double the distance recorded on the watch (ex: If the watch records 1.0 miles, the calibration value input must be between 0.50 and 2.0 miles).

That might explain what happened to you but I’m not sure the math adds up in your case. That’s the only info I could find about limits to the calibration. You should still be able to edit the distance without limits afterward on the app (does get annoying though)!

The “Indoor Walk” activity doesn’t let you calibrate the distance on the watch, but “Treadmill” does if you walk over 1 mile. And “Treadmill” does actually learn from each calibration and should get more accurate over time