I'm not too sure if an average team speed is what your looking for here. After all, it doesn't help you build a team if I say that a team's average speed is 144, but the team contains 3 Pokemon with 90 speed, and 3 with 178. If you went with some +Speed Base 80s, that have 145 speed, in order to one up the opponent's average team speed, you'd end up in the middle of nowhere.
As such, information on where individual common Pokemon usually end up speed-wise is more important, as opposed to averaging things out. You will often to see a lot of Pokemon have around 80~100 speed, 132~145 speed, and 167~183 speed, excluding Trick Room Users/Pokemon, Scarfers, and Pokemon under the influence of Tailwind, Swift Swim or Chlorophyll.
Hopefully this helps.