A brisk but comfortable walking pace for me is 4.5mph at 6% or 7% incline. Comfortable, sustainable running speed is 5.5-5.6 mph at 1%-2% incline. Fastest? Doing treadmill intervals I can do 8.1mph at 1%-2% incline for 30-45 second bursts.

Boy are my short legs a going like crazy when I do this! lol