Many years ago when I was in training, I used to do an hour on the stopwatch and keep a record of my distance covered. My best was 12.95 mph, which is 20.83 kph. That is 20.83 actual km covered in an actual hour, so my peak speed was higher, but not recorded.
That was on a steel-rimmed Coker Big One with 152 mm steel cranks, and no seat handle or bars.
I couldn’t do it now. I struggle to even hit the speeds I used to average for sustained periods.
For comparison, my big ride last week, 51 km on my KH36, on varied roads, average speed over that distance was 9.93 mph = 15.97 kph.
My peak average measured over the fastest 1/10 mile section of that ride was 11.55 mph = 18.58 kph. As that was an average, my absolute peak speed within that 1/10 mile section was presumably a tad higher.