I am having this same problem, but I switched from the Garmin Forerunner 35 which was treadmill accurate, to the Forerunner 235 and now it says I run 7+ miles when the treadmills shows 5... even when I manually entered my stride length i did a 2.6 mile treadmill run and the Garmin showed 3.8. Do we have to calibrate each watch outdoors before it will work properly indoors?
yes you will want it on auto-stride length and run outside a couple times. Ideally on a fairly flat area with good footing (not off road or on snow/ice). Sounds like you run indoors typically? Good to do the run at around the paces you typically run on the treadmill... and a few other faster/slower paces for a while. This is so it can create an index of your pace and cadence relationship at various speeds.
Actually, i normally run outdoors, but my running partner had a knee and wrist injury a few weeks back, and generally prefers the gym anyway since we are different pace. My previous Garmin had no problem with the treadmill, and it was set to auto stride length. I recently got this new one, and assumed the stride data would transfer to this watch, but you are saying that is not true? That seems really inefficient.