So...does it have a 4-banger or a 5.0L in it now? I'm confused, sorry.
I'd check for obvious stuff like vacuum leaks, misfiring cylinders, bad ignition system parts (plugs, wires, cap, rotor, etc.), that kind of stuff first. Is your check engine light on? How about the EGO sensor? Read your plugs and tell us what you see. As a check, fill your tank completely and see what the gauge reads. 4-banger and 5.0L gauges aren't different to my knowledge though. Good luck.
------------------
Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch