56.5 mph on the Carson Pass descent into Woodfords during the 2009 Death Ride. I didn't have a Garmin yet but a Polar CS200 HRM. I didn't look down at the time but thought to myself this was really really fast and probably faster than I had been on a bike before. I rode my Jamis Eclipse with a Campy triple that day. 53x11, pedaling full steam on the flattish section to dump every one on a compact behind me. Once you get past the 88/89 intersection you can really dial up the speed as the road dips quite a bit. Two taller riders passed me up near the bottom so they were probably hitting 60mph.

My wife and I on the tandem can easily hit 50mph without much effort though the practical side of me says don't push it much past that.