1) On a treadmill your actual effective MPH or feet vertical climbed is less than reality due to the ability to "cheat" a bit by "bouncing" instead of STRIDING and Accelerating.
2) This is also true on a stair climber, but less so as it mimics steeper grades more effectively.
I have found a treadmill at 15% grade and jogging at say 6mph is equivalent of trail walking at around 4mph of the same gradient.
About 2/-3/4 the actual "numbers" read on the treadmill. You also do not develop your balance and stability muscles that are very important for any uneven surface.
If at all possible I try to get out on a real trail, even urban trails. Doesn't happen nearly as much as any of us would like as it simply inconvenient most of the time.
PS 15% grade = 15 feet vertical for every 100 feet horizontal. Or roughly 750ft vertical/mile. (15% grade is not an angle of 15 degrees.)