What's wrong with my car? Bad fuel mileage!?

Bought a 2007 Ford Focus in June (08) with 31,000 miles. Since the day we took it home, we only averaged around 18 mpg. (I do primarily city driving which was the reason I chose the Focus). EPA was 27 - revised EPA is 23 but everyone says they always see high 20's.

The Ford lot here says they can't figure it out. They tested it at 17.8 mpg at 35 mph and agree that it should do better than that. They basically said they don't know why my car runs as good as it does because losing that much mileage, it shouldn't.

We recently took it back to the lot we purchased it from and the claim they did "nothing" to it - yet "it" tested at 26.9 mpg on their computer. When we got it back it did do a little better - 24 mpg, but dropped off after that (21.19, 20.27, 18.8).

Just curious if anyone has had this problem so we can try to pinpoint it.