Believe it or not, when you add a dirt section to Calle Monroy which more than doubles its length, the average gradient actually increases! I have no idea whether this thing is rideable or not. Probably not.
Calle Monroy (w/ Dirt Extension) is located in Tenerife. It is 1955.7 meters long with an average gradient of 27.3% and a maximum gradient of 35.9%. Find similar hills.