You got me beat, the steepest I have seen a sign is 14 percent. Part way up this hill, I decided that I did not see any cars so I created some of my own switchbacks back and forth using the full width of the road to get a breather for a short segment.



I calculated 3.0 mph at 15 percent and assumed no aerodynamic drag and no friction, all effort went into elevation gain. (I had already set up a spreadsheet to estimate how long it will take to climb a specific hill that has hour restrictions this summer when I cross the continental divide, thus I only had to plug in the numbers.) Where did you get a formula for aerodynamic drag?