How agressive are we talking? If you shift at 5k rpms at every on-ramp, you'll burn fuel fast. I shift at 3-4k rpms and get over 30 mpg. As mentioned before, 20 sounds like an actual systematic problem. If you've got an ef, you could have a terribly clogged fuel filter, which would bring it down some, but not to 20. Try replacing it--it's a good place to start, and you'll get a chance to check that all your other hoses are hooked up while you're in there.