maybe your alignment is off? there are so many different things that can effect gas milelage, like everyone else said, check the plugs for right gap, wires, and everything electrical....fuel pump is pretty easy todo and maybe the airfilter?

on this matter, if the spark plugs are really soot-y.... does it mean its running too rich? and if your car is basically stock... are you able to adjust the lean/richness? or is it controlled by the ecu