51 mph on the east side of Carson Pass between Sorenson's and Woodfords. Done that descent on several tours and usually could hit at least 50 where it was straight for what seemed a mile.
50 on the east side of Tioga Pass, once, but that was a bit scary, what with some random rocks in the road here and there. Most of the descent I stayed at 40 or below.
50 once going down the west side of Ebbets Pass into Hermit Valley where the quite narrow road ran straight for a bit, escaping a thunderstorm at the Sierra crest and having its downburst as a tailwind. That was pretty hairy--a van was several hundred yards ahead of me and disappeared around a corner. I began slowing quite a bit when it did that, and a good thing, too, since when I came around the corner, it was at a near dead stop as it edged by a large pickup coming up the grade.
Locally, I could usually break 50 coming down into Slaughterhouse Canyon on Hwy 67 near Lakeside in San Diego County, coming back from an overnighter at Cuyamaca State Park's hike and bike sites. Warm temperatures, a normal afternoon tailwind, some draft from any passing vehicles, and a big gap in traffic to get off the wide shoulder out into the right lane of the two downhill lanes to be free of any debris on the shoulder would play a big part in hitting that terminal velocity. The speed limit was 50 on that road, so traffic wouldn't overtake me too soon, and the regular drivers knew that the CHP often sat at the bottom of the grade clocking drivers, especially where the two lanes would merge into just one and idiots would race other cars just to be first into that lane.
53 on the shoulder of I-8 about 5 miles east of Alpine in San Diego County during a rather major September offshore wind event. I had to continually brake when riding the 4-5% grade down through Alpine to keep from breaking the 35 mph speed limit there that day--way too dangerous with all the business cross traffic there. Temperature was 113 F by the time i got down into Lakeside--phew!
Plus a mirror to watch overtaking traffic on all these hills, of course.