Just thought I'd pass along a bit of self-acquired wisdom. It may apply to more than just civics.
My 1997 Civic EX (SOHC V-TEC)[D16Y8] was averaging between 24 and 26 mpg. I knew there had to be a problem somewhere since it should have been much better.
Changed out spark plugs, wires, air filter, fuel filter, AT fluid, etc. None of it worked, so I said the hell with it..
A month went by, and then I decided to have a go at it again. I scanned the honda newsgroups and I can't remember who suggested it to someone else, but it ended up being the PCV.
The PCV! A $2.75 part boosted my milage to between 31 and 33 mpg.
I had a hell of a time finding it on my car. On a 97 Civ EX, you have to get under the car and look above the oil filter a few inches towards the passenger side. Even the picture in the haynes manual is vague.
Another milage tip: Keep your tires inflated to the pressures shown on the sticker on the driver side doorjamb.
-Marco