As an example, I'll pass on a recent work 'watercooler' conversation I overheard.
Man1: So, did you bike to work this morning?
Man2: (Dressed in cycling gear) Ummmm, yeah...
Man1: Do you bike to work much?
Man2: Only on Friday, sometimes when the weather's nice.
Man1: So how far do you commute?
Man2: 20 miles, one way
Man1: Does it take long?
Man2: Oh, takes me about an hour.
This made me think about my commute. Total distance from the trailhead to my work is about 13.5 miles. I usually get to the trailhead a bit after 6 a.m., by the time I get the bike unloaded, bags on bike, and geared up I'm probably off by 6:10 or so. I crank out my ride, get to the parking garage, lock up the bike, unload my stuff and carry it to the gym. It's around 7:30 at that point. So taking into account gearing up, loading, unloading, and walking to the fitness center I'd say actual riding time takes 'about an hour'.
If I round mileage up, I could say my ride is 14 or even 15 miles. Or I could round time down and say my ride takes 50 minutes or so. Even then, I'm still only riding a 15 m.p.h. pace at best, fully loaded. So when I hear 'Man 2' telling his pal about his 20 m.p.h commuting pace it makes me wonder:
-Which direction was he coming from?
-Did he have a tailwind?
-Is his work commute all downhill?
-Are there no stop lights or stop signs? No crosswalks? No traffic?
-Was he towed? Did he hitch a ride part of the way?
-Does he have an electric motor assist on his bike? Gasoline engine? His 'bike' is really a Moped?
-Huge thighs and quads? Massive 53 x 11 gear? Low rolling resistance tires? Aero bars?
I mean, it's not inconceivable a casual cyclist could run a 20 m.p.h average into work. Let's see, yeeeah, I ride all the way from my house to Omaha every day. Yeeeaah, that's about 35 miles or so and yeeeeah, I can do this all in about an hour and a half..including my shower. Yeeeah, that's the ticket!
Sounds impressive, doesn't it? All it takes is a bit of approximation :>)