pace/distance discrepency

My Venu 3 watch is recording a much different pace/distance for treadmill runs than it does for outdoor runs.  Is there a way to fix it?  My pace for outdoor runs in under 12 min/mile which I know to be accurate, but on treadmill it is recording over a 15 min/mile.  I see that I can manually enter my stride length to help, but I have no idea how to do that.