Yeah, I did want to mention again how much the terrain makes a difference. I can do a relatively flat ride to the beach and back at a 20 average, (the only time I have gotten a 20 and it's a total thrill ) or I can climb 7,000 feet and get a 12 average.

A lighter bike, clipping in and pedaling efficiently make a big difference. Keeping your tire pressure up (i.e. pumping before every ride ... making sure you don't have a slow leak and/or brake pad rubbing ). The weather, fueling, etc. All of these things play a factor in speed in addition to your strength and ability level.