So from what you guys are saying, I just need more time with the gearing I have and work into it. And the better I get at riding, the closer I'll be to optimizing my average speed with the bike the way it currently is set up. Makes sense, and is simple enough to understand.
At the very least, it seems like I'm right in line for my level of experience. For the most part I was between 16 and 19.5 mph where I was riding.
Plus, I'm not completely comfortable on the bike yet, either - I need a new seat or something, because this one sucks donkey balls.