This is an interesteing tid-bit from your site (
http://www.geocities.com/sidestreetluge/grade.html) I did not know...
Q) Why use percent of incline vs. degree of angle?
A) The reason for using percent of incline instead of angle, is that percent gives a direct way to assess the effort required to move forward against the grade, whereas the angle in degrees does not readily reveal this information. For example;
a 5% grade requires a forward force equal to 5% of the weight of the
object (above and beyond the force it takes to overcome surface resistance on flat ground at the same
speed).