I think I stumbled upon the steepest mile of climbing in Southern California. I was riding up Yerba Buena when a little after two miles I saw a very steep road on the right called "Yellowhill Road". I decided to give it a try. Wow, what a surprise! Immediately hit a 15-17% grade which just kept on going and going! I kept wondering when the road would stop, but it didn't! My garmin read 20-22% on a few occasions. Grade didn't let up until the top. The road ended up being 1.06 miles with 830' of gain, for an average gradient of 14.8%. I don't think I know of any mile sections in SoCal that gain this much elevation.
Proof - look at split #9:
Yellowhill on Google Maps:
According to "The Complete Guide to Climbing (By Bike) in California", the steepest mile of climbing in the state is Mix Canyon miles 2.8-3.8(15.8%). The second steepest is Old Priest Grade miles 0.3-1.3 (15.0%). This qualifies as the third steepest mile of climbing in the state of California.