22mpg is kind of rediculous, I actually didn't think a Civic could get that low. Something is definitely going on there.
Well you should notice a small improvement by
inflating your tires which can give you a few percent gain. Are you throwing any CEL? Might want to check your cat as well... I had that problem once, with no CEL.
You may want to consider
slowing down a bit as well. I don't think the difference between going 75mph and 60mph on a 18 mile trip is going do make a dramatic difference in the amount of time it will take you to get to your destination, maybe a few minutes.
There are plenty of mods in the
65+ Efficiency Mods section that you can look through to see how extreme you are willing to go with your car, but that car with the right mods and driving habits can achieve some incredible numbers. Just look at the left side of the ecomodder homer page and you will see that 8 (5 being Civics) of the top ten mpg vehicles for the site are Hondas.