The maximum speed of treadmills varies according to its quality and purpose. The highest-end commercial treadmills used to train elite athletes top out at 25 mph, while the treadmills in your fitness center will reach speeds between 12 and 14 mph. A treadmill purchased for home use generally has a maximum speed of 10 to 12 mph.
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Go as Fast as You Can
A less common, but inexpensive option is the manual treadmill. These are lighter and smaller than motorized variations -- the catch is, they only go as fast as you do. You may find it hard to get the belt moving initially, and the belt's movement tends to be less smooth than a motorized version. The belt does stop when you do, so you don't have to worry about road rash at 12-plus mph.