my 91 fireturd does the same thing, it used to get good gasmilage, (before my mods) now it sux big balls. I replaced my O2 sensor and all is now ok. try that, its only 21.99 at advanced auto , or pep boys. its either that or the converter is clogged.
Do a tune-up. I would bet money that if you pulled off you distributor and checked inside that all the contacts are corroded. Buy a new cap and rotor (pretty cheap) and you will notice a pretty good difference. I have gone through this with my 91.