I think my low has been around 23-24mpg...that would be in the dead of winter, 100% city driving getting stuck in traffic and playing in the snow.
How are your tires; do they look like they are wearing evenly, properly inflated and a good alignment? That can hurt mpg's a little bit as well. Something you can look at but i doubt it would be the main reason you are seeing such sub-par mpg's.
You need to run several tanks of gas and then see about getting a good baseline for mpgs.