Rush hour traffic
8am, on Airport Blvd in Mobile, AL - seems like the lowest mileage any car will get will be on this road. 4 minute stoplights. I hit at least 3 of them along my 4.4 mile route. Stop and go traffic approaching the lights. I've gotten 23mpg some days, per my Scangauge (avg speed 12mph). Those days I have to accelerate hard to keep up with the big trucks behind me (they WILL bully the small one in front or zip around you if they're late for work, which they usually are).
But today I went in to work for a special project, same exact route... I got 43.1mpg (city!), avg speed 21mph... so the stop and go traffic makes a huge difference. On my first highway trip through 4 states, my first tank was 37mpg, second tank got me 44mpg. It'll be better next month after I've learned to drive more smoothly.
So, 23mpg city is my worst so far... and I know the reason for that is the traffic. The same traffic and route in the Explorer gets me 8mpg, so I really am happy with my purchase. Sure, a Prius could probably get better, but I like this little thing. The soccer ball sticker I saw on a Prius just killed it for me. Please, for the love of God, nobody put a soccer mom sticker on a smart!