Treadmill mis readings

You can't have an accurate distance on the watch without GPS...the distance is calculated with the movement of your arm and the length of the step. If you move the arm quicker or slower the distance will adjust accordingly...if you check the watch for 10 seconds you're already changing the distance that the watch is recording.

Which treadmill are you using? 

To get accurate treadmill distance you need to use a foot-pod device or an NPE Runn sensor to put on the treadmill. I use Stryd and NPE Runn. Stryd is for me the NPE Runn is used by family and guests wanting precise measurements and is mounted permanently on the treadmill.

Most treadmills are not very precise when showing distance. Every time your foot hits the TM surface - you slow down the belt causing it to slip on its rollers. The slip causes the TM to show you have traveled further than you have in reality.  The more "built like a tank" your TM is - the less of an issue it normally is. But even the best TM's need a lot of service and care to show correct distance. 

There is only two brands I have gotten consistent and reliable distance measurements from and that is LifeFitness and PreCor. But both in well maintained condition. Nordic is supposed to be good as well but I have not tried myself. I have tried quite a few "Italian" brand TM's and they show distance like the wind blows even on the most high end versions in commercial gyms.

If you do any significant amount of treadmill training, this sensor is worth the investment. It connects to your watch as a footpod.

npe-inc.com/.../

Thank you for your response. I understand that the reason to record the length of your step is to calculate the distance walked or ran, actually the steps are recorder accordingly.  The watch is set to get connected to all the GPS available, but since the treadmill is indoors could interfere with the reception.

When you select an indoor activity like it can be the "treadmill run" gps(satellites) is not even taken in consideration as it is off and not selectable. GPS(or more generally satellites) are used only for outdoor activities.

Steps too are recorded with the arm/wrist movement and just because you see the number increasing it doesn't mean the amount is correct.

Have you measured and set a custom stride length within the Garmin Connect app? (Settings > User Settings > Custom Stride Length)

Since doing this my treadmill workouts have been a whole lot more accurate.

Well, this is all an opportunity for improvement, since I certainly not satisfied with this that for me is a limitation, Regards

Well, you can't use satellites in a closed environment...there's no room for improvement on that. Also if we consider that you can use them, you're running on the same spot so the result would be 0 meters 

And measuring the correct distance with the arm movement looks pretty impossible if you're not a robot and moving them exactly in the same way for every step that you're doing. You should also avoid to check the watch during the run.