Are you really aggressive on the accelerator/brake? Are you driving a lot in a downtown area where you're stuck at a ton of stoplights, accelerating, and decelerating a lot?
A lot of engines do take time to work themselves "into shape" if you will, moving parts adjusting to each other with time and getting small bits of production residue/metal out of the way, so that could play into it too, but if you're burning through a full gallon of gas in 18 miles, I'd guess driving style or number of stoplights plays into it.
At worst, if I was driving around in Downtown Pasadena for groceries only with my Civic, no highway driving, I'd wind up around 20-23 miles per gallon because there were so many stoplights, and it's rated at ~40.