I had no idea how tough hills could get when I tried Brasstown Bald back in 1998. I showed up with a 53-39 (no one had heard of compact cranks at that point) and a 12-23 cassette. It didn't work out so well, and I walked several large segments of it.
Brasstown Bald is located in Georgia. It is 4782.2 meters long with an average gradient of 11.3% and a maximum gradient of 17.7%. Find similar hills.