Originally Posted by MaximusFunk

might wanna try a valve lash adjustment. do you hear a ticking sound for a few minutes after you start your car, even in warm weather?

valves out of spec can cause bad gas mileage, loss of power, and rough idle as you mentioned.

Also, might wanna try some gumout complete fuel system cleaner to clean out your injectors and intake valves. cant hurt to try for the price.

Also, is there a lot of crap in your trunk and back seat? more weight equals worse gas mileage. a couple hundred pounds of junk in a civic can make a big difference.