replace the PCV valve, give the car a tuneup.. 31mpg seems kind of low. I can guarantee that the PCV valve is clogged with gunk and has never been changed. If you give the car a good thorough tuneup, including adjusting valve lash, I don't see why your fuel economy won't go up.
Going by the fuel economy numbers of the freestyle and a Civic EX manual, fuel economy should be about 29% better in the civic than the freestyle, so you should have gotten around 36mpg assuming everything else is the same. Like I said, car needs a tuneup. Problem with me saying that, is that a tune-up means different things to different people which is annoying because a tune-up shouldn't just be plugs and wires like some folks think.