My brother and I have a joke. If someone says they can do 30, they can really do 27 consistently. If someone says they were commuting at 20 mph, they mean 17 mph... Everyone will overestimate.

If you're talking about really pushing on the flats, a good non-pro rider can hold 30 mph for a short time.

If you're talking about time trials in the category 3-4-5 level, 23-27 mph is about the sustainable range for winners of races more than a half-hour.

Try looking up some TT race results for flat courses and calculate their average speed.