Joined: Jan 2009
Posts: 44,178
Likes: 6,405
From: New York, NY, and High Falls, NY, USA
Bikes: 1962 Rudge Sports, 1971 Raleigh Super Course, 1971 Raleigh Pro Track, 1974 Raleigh International, 1975 Viscount Fixie, 1982 McLean, 1996 Lemond (Ti), 2002 Burley Zydeco tandem
[MENTION=200675]chas58[/MENTION] is right. The difference between average speed and cruising speed is important. This accounts for lies, exaggerations, and mistakes. Maybe some people look at their speed readouts only when they're going their fastest, and this misleads them into thinking that this is their average speed.
20 mph average is nearly inconceivable to me. I know it's possible, but I expect it's very rare. You'd need to be pretty darned good and also have really good equipment and also be lucky enough to have very clear roads with no lights.