Myself nor my teammates go by the average speed calculation. It's really not needed at a certain point. The only thing we use when talking about pace and speed is the speed we actually ride at. Big difference between that and calculated average speed on your computer. For example, we'll go on a 35 mi ride. On any given road we're between 19-23 mph. However when we get back to the bike shop and get off our bikes, the computer may say that our average speed was like 17.5 - 18.3. Big difference. So we just go based on what speed we physically ride at. It's easier and better anyway because like everyone has said - terrain, wind, temp, elevation, lights, stop signs, etc. - all affect your speed.

That's my 2 cents