When putting in many miles with the Wahoo Stride Sensor the calibration can change from day to day. It is generally plus or minus about 3%, but it might be perfect today, 3% fast tomorrow,and 3% slow the next. That's where the autocalibrate comes in. What it does is utilize the GPS reported workout distance to constantly adjust the stride sensor calibration. GPS is generally very accurate at accumulated distance, but horrible at instantaneous pace. So with auto calibrate on you get the best of both worlds super accurate instantaneous pace, corrected for accuracy by the accumulated distance from the GPS.