I don't know what the steepest grade I have ridden up is. I know that I rode up the back side of Gates Pass in Tucson, AZ, in a 93" gear, sitting down, in my long-gone youth. Longest climb would be Mt. Lemmon, 29 miles, 7000 feet. Sunrise Drive in Tucson was wicked-steep, before they made cuts in all the ridgetops that it went over.
Now, in my geezerhood, there is an overpass climb that I estimate at 11% for about a thousand feet. It is a challenge now, for me at least. My lungs give out before my legs do, but I can make it on a good day with the 28/28.