Bin Der Dun Dat wrote:
http://www.hillrunner.com/training/tmillchart.phpHere ya go. I can't vouch for the accuracy of the chart, but if it's posted on the Internet, you can be pretty assured that it's legit ;-)
The first problem with that chart is that it assumes equal running to roads at .5%. It then knocks off huge amounts of time on the initial % changes but keeps reducing it as the level goes up. It also doesn't take into account the cumulative effect of the build up of heat on a longer run that will send your heartrate skyrocketing and make the level much more difficult. That is why the treadmill may feel easier the first half hour, but harder the next half hour. Another factor it misses is the weight of the runner. A heavier runner will have a harder time overcoming gravity than a skinnier runner. I am pretty light and have always run the uphills better than most.
For me, I believe the difference is to add 20+ seconds per mile per 1% incline.
I have been working out about a month at 12% incline between 11 and 12 minutes per mile. (helps my back) I did my first run outdoors in 3 months (had a broken toe) and ran a hilly 5 miles at 7:05 pace.
Today I ran 11:45 pace the last 7.5 minutes of an hour run at 15% incline and it was much harder than the 6:30 pace I ran the last 9 minutes of the outside run. It would have been closer to 6:10 to 6:15 pace so that would be 5:30 difference for 15% or 22 seconds per 1% incline.
My experience with the treadmill is that a lot of running at level incline puts more stress on the quads, (like downhill running) and I had knee pain all last winter until I got back on the roads consistently.
Another way to look at it is cross training. Instead of trying to compare to outdoor running, just compare each tempo on the treadmill to the next tempo on the treadmill and it will be a much easier comparison.