I have been known to take liberties with mileage estimates but not more than 10 miles.
One of the places we like to vacation and ride is Cambria, CA.
From there you have easy access to Big Sur and other nice places.
One day we decided to ride inland from Cambria on Santa Rosa Creek Rd.
The road started out flat and gradually became very narrow, windy and highly scenic.
The road was sprinkled with Trantulas (live) which is something I have never seen before or since then.
Also we saw flocks of turkeys along the side of the road.
We rode through a tree tunnel along a creek.
The further we went the steeper the road became.
Finally we came to a wall of a hill. There was a traffic sign part way up the hill and instead of it being vertical it was slanted because the hill so steep. I shifted to our lowest gear and we pushed as hard as we could and then came to a stop.
My wife was in tears because this hill was so intimidating. I had no choice but to abort and turn back.
A few years later we returned and I rode the same route on my single.
When I came to that same hill, it took everything I had to get up it.
I think it must have exceeded 20%. There were plenty more steep hills after that until I hit Hwy 46.
Another climb on Hwy 46 to the summit and a then long screaming downhill back to coast Hwy 1.