Recorded distance versus treadmill distance

Calibrate the watch and see if the discrepancy narrows

Hi, thank you for the response. I've already calibrated it several times and rather than improving it is getting worse

If you’re certain the treadmill is accurate and that you are calibrating the watch properly then perhaps you need to open a discussion with Garmin Support. 

Dumb question maybe but are you swinging your arms freely the whole time on the treadmill? I use the treadmill a lot and the Garmin is pretty close to what the treadmill says.

It is based on an accelerometer. If you calibrate but , later, you move the arms differently (faster, slower) it will differ from treadmill distance. 

In my case, in 10km, "error" is 100m max. 

Use hrm pro plus and opt out indoor only for accurate results.

Since the gap is way too big, there may be an issue related to the way the watch estimates your stride length. Please check your profile settings (height, weight, age etc.) and retry. You must have your complete profile data properly defined on the watch.

I am experiencing the same issue. And, for some reason, even if you get "more or less" a good measurement, after connecting the watch to the PC for syncronization, the next workout (same pace, same movement, etc) gets worse again.

Going to your question, in my case, running at 4,36min/km (13km/h) I can see 4.4X and 4.30 randomly. Never hoilding handlebars, consistent moving, calibration after each workout, even if, luckily, distance recorded is accurate (+/-100meters at the end)

Maybe you could consider calibraiton not only when you finish your workout, but also in pause menu, as an intermediate calibration.