I manage 17-18 mpg. Oddly, whether I'm driving city or highway, it doesn't make a whole lot of difference. That being said, I do drive with mpg in mind. I don't race to red lights like most people do and when possible, I try to time lights so I'm not coming to a complete stop.
On the highway, I've noticed that speed makes a HUGE difference. I frequently drive the Coquihalla Pass highway in BC. Keeping a steady 120 km/h, I average around 15 mpg. If I keep my speed under 110 km/h, it jumps up to 18+. There could be other variables in there that are exaggerating the difference, but either way, it does make a significant difference.
I just removed my factory roof rack for the sake of reduced drag. I realize that it will probably make no noticeable difference, but it's free and I figured why not.