"My impression is that the ""15"" in B15 indicates a ride's speed on the flats. Given that most of us slow down a bit on the hills, the general rule of thumb is that average speed will be 3 mph below speed on the flats. Your mileage may vary, of course. But relativistic effects aside, average speed is distance divided by time.

Of course, in bicycle computerland time can be calculated in two different ways. Most computers can be set so they turn off the timer when the bike isn't moving, so your computer either calculates average speed using total average time from start to finish, including all stop lights and that muffin break at the Runcible Spoon (usually a discouragingly low number, and not too diagnostic) or it tells you your average speed during the time the wheels were actually rolling (more encouraging and better for gauging this ride against your last one).

I think that's a more or less accurate explanation, if not the clearest. Someone please let me know if I've thrown my chain anywhere along the way."