50+ mph drafting a truck in Mexico during the 1980 Rosarito-Ensenada on a winding downhill, according to friends watching my stupid stunt. Hey, I was in my early 20s and invulnerable. Not something I'd do again.
On solo rides, no drafting, 40+ mph several times on some local -2% downhills, depending on tailwinds. Nowhere near the fastest times. Those were set when that segment was closed to traffic so cyclists could build up a good head of steam before the segment. Now there's too much traffic and the intersection is blinded by trees, so I don't blow through it. From a dead stop at the intersection I'll usually peak around 35 mph. Tougher now that the road crews chewed up perfectly good smooth asphalt and laid down the roughest chipseal I've ever ridden.
43 mph on a -3% downhill, with a tailwind assist. Also before that road had much traffic. A little harder to do now due to three lanes converging into a bottleneck entry to a bridge, due to a stupidly designed road. Gotta keep your head up to watch for indecisive drivers who might let you go ahead, or might try to squeeze you into the bridge abutment. Harder to stay in an aero tuck now. I wish they'd blacktop that hill. It's striated concrete now with some cosmetic bricked intersections. With smooth blacktop it'd be easy to reach 50 mph on that downhill on a good day with a tailwind.
The only fastest times I'm really concerned about are sprint zones on some false flats that are sheltered from wind, so no tailwind assist. I never considered myself much of a sprinter but working on it this year surprised me. I still ain't fast but I'm faster than I thought I'd be for an old man.