Get a tornado in your intake man!
Actually I was just kidding. I was going to say check your o2 sensors. My Eclipse that I had was getting terrible mileage I mean like 18-20 mpg in a non-turbo 4 cylinder. My check engine light didn't come on either, but it turned out my 02 sensor off of my downpipe was burned out, and the computer just reset itself so it didn't throw a check engine light.