On my bike commute to work today I averaged 16.9 mph and arrived in a record time. On the way home it was 12.1 mph. The difference? 19 mph headwind on the way home. The gusts were up to 25-30 mph. Yuck.
I'm not a fast rider... at all... seriously. But 12.1 was a little rough. For the first 6 miles it was about 10 mph. So I picked it up after turning out of the wind.
But I would never trade that bike commute for a car!
Get out there.