When running indoors or on a treadmill, it gives a distance measurement error of +40%.
There is no calibration function when saving. How can this be fixed?
For treadmill there should definitely be a "calibrate and save" option in there. You might need to scroll down to see it. I don't know of anything like that for indoor track running.
Forerunner 265 Series Watch Owner's Manual - Calibrating the Treadmill Distance (garmin.com)