On longer rides I try to pace myself at 16.5mph but then comes a turn into the wind and everything goes out the door. Headwinds are the great average killer. That's why avg's really dont impress me much. Cruising speed however, is a bit different. If someone asked me what I did on the flats with no wind I'd tell them 19mph or so but it's not my distance pace.

It's complicated and really needs to be resettled on every ride