The longer the ride, the lower the average speed. I used to reset the computer on my commuter bike, which is a folding bike, at the beginning of every month. When I got to the train station on morning of the first work day of a new month, my average speed was usually around 15 or 16 mph. Then I'd fold up the bike and get on the train... get off in the city, push the bike along the platform to the escalator... and my average speed would have dropped a little. Ride to the office, it goes up again. But by the end of the month it was always down to about 12.3 mph.

Similarly when I ride a century, I can watch my average speed start out around 15 or 16, but by the time I get home, I'm lucky if the 'moving speed' is over 14. Total average speed, for a century, is never much over 10 mph.