I know when you begin riding you fixate on average speed. Like others have said, there are too many variables that have to do with average speed which makes this metric less useful. Just note how fast you usually go on a particular section of a route and note how you improve over the course of weeks and months.

In my commute route early in the morning when I can blow past all the stop signs and red lights I can average 2+ miles faster than the homeward ride when I obey all the traffic laws.