First, we all know there is no wind resistance. How much is that REALLY costing you on your run? The information I can find says it is anywhere from 5 to 10 seconds per mile at 4:30 to 5 minute pace. How many of you are routinely running that fast on the treadmill? As the pace slows, the effect of no wind resistance decreases.
The opposite is true when you calculate the effect of incline on pace. At 5 minutes per mile, 1% incline will cost you 12 seconds. While very close to the upper limit of 10 seconds per mile saved from wind resistance, it is still greater. If you calculate 7 minute pace, the cost of wind resistance is lower (maybe much lower) but raising the incline 1% will cost you 15 seconds per mile. At an easy pace of 8:30 per mile, the cost of wind resistance is said to be negligible but raising the incline 1% now costs you 17 seconds per mile. That easy 8:30 is really being run at 8:13 effort.
The other factor involved that can make a huge difference is heat dissipation. The lack of air movement and high temperatures of some fitness centers can create an environment equivalent to a dewpoint of 70 degrees. At that level, pace will slow by 2% to 3%. For 5 minute pace, that can be 5 to 9 seconds. For 7 minute pace 8 to 13 seconds and for 8:30 pace 10 to 15 seconds. While heat dissipation may affect you very little on shorter runs, it will make a bigger difference the longer you run.
Considering the above, if you are gifted enough to be routinely running 5 minute or faster miles, then raising the incline 1% will give you a close enough effort to running outside on a completely still day on a flat surface or you could just bump up the speed a few seconds and get the benefit of faster turnover.
For the rest of us slowpokes, you can just keep the treadmill at 0 incline and run whatever pace feels right that day and it's going to be pretty close to what you would have done outside running around a flat track.
Of course we know that very few people just go run around a flat track all the time so feel free to hit that incline button all you want and mix in some hills. Just remember that every 1% you increase, it will be 10 to 15 seconds harder for the fast runners and 15 to 20 seconds harder per mile for the slower runners. If you really get ambitious and crank up the incline into the double digits, it will get incrementally harder depending on your pace and when it becomes more efficient to walk rather than run.
Tomorrow I will analyze the difference between running on a flat track with a 7 mph wind at our back, a dirt trail with some roots and rocks at an average incline of 11%, and a descending paved road with a decline of 6%!