Yeah...I´m fairly sure it has to do with the fuel delivery. Once I got my air intake and exhaust pipe fixed I was still only getting 19 mpg...it has to be something to do with the fuel filter, lines, rail, injectors or a combination of all of them. Try running some cleaner through them. or if you really to fix it, go get a new filter, lines, rail and injectors :-D
What year is your car? What I really want to know is: Does it have the Ford EEC-IV computer (Up to about ´93, I believe), or does it have one of the other PCM´s? (Powertrain Control Module) It could also be EEC-V or MECS (Mazda Engine Control System).
Once you know what you have, it´s easy to get in and check for trouble codes set by the computer. On an EEC-IV, all you need is a jumper wire, and an analog multimeter if you don´t have a "Check Engine" or "Service Engine Soon" light.
Let the computer tell you what´s wrong; this will save you a lot of time and money, and frustration.