Out on my ride yesterday I thought about this post and figured I would just ramp up on one of the flattest sections of my ride and see what I could hit. We really don't have any "flat" sections of road here. This one strava segment is -0.7% for 2 miles.
@47 years I was able to sprint up to 31mph with a 10mph headwind on a Ridley Helium outfitted with Ultegra.
On this same 2 mile segment I can average 26mph. Typically on this 20 mile ride I will average between 18-19 mph an that includes about 1200-1400 feet of climbs.
So many variables though if you are not on a track or some closed course with zero wind. And of course without the use of a properly calibrated device to measure our top speed we may never know exactly what that is. It is nice to have the electronic tools of today to have some semblance of measurement.