OP, you are misunderstanding. Few here are trying to give you bad info, but they are saying it is very difficult to have meaningful data across a broad spectrum.

For you to use avg speed to compare yourself to yourself on a same ride, it is perfectly valid, though temperature and wind will affect it. But you'll know that, because you are experiencing it directly.

A 12-13 mph average for a recreational cyclist on a recreational ride in average terrain with normal winds and typical summer weather is within the bounds of average.