The US EPA estimates gas mileage based on lab tests, not real world driving.
People with a heavy foot and lacking the habits of checking air pressure in the tires, changing oil at regular intervals, etc etc will fare worse.
Sea altitude affects gas mileage too. I don't know where DR stands on this front, but the higher altitude, the lousier your mileage will be.
I drive a Honda Civic that's supposed to get me 28 MPG city but I usually average 22. Stop and go traffic here can suck.
