There are so many things that factor into average speed... it just does not matter. The proper way to measure your improvement is by power. Here is an example:

One week you ride on your favorite road with no wind. You average MPH was say 18... but your power output is 180 watts avg.

2 months later you ride the same route but with a tail wind. You average MPH is up to 20 MPH, but your power is 160 Watts. You did not ride as well as the first ride.

2 months after that, same ride, now a head wind. You average 16 MPH, but your power output has increased to 200 Watts. That shows you have made an improvement in overall power output. That is a real benchmark of performance.

Wind, road conditions, stops, air pressure, body position... all these things can be factors in Average MPH.

MPH IS NOT AN ACCURATE benchmark of performance! End of Story.

When my computer gives me my AVG MPH is includes the warm-up and the cool down. I am putting much less watts during that time, trying to keep my heart rate lower than when I am on my ride.

Oh yeah... and the only way to really improve... BASE MILES!! And that takes (they say) 3 -5 years to build!