My car is a 2001 Cavalier 2.4L 4spd Auto with roughly 55k miles, the last tank of gas i got around 19mpg-20mpg approximatley. Now for the latest tank of gas I got the samething but I did these things:
changed fuel filter
added STP injector cleaner
checked the throttlebody for dirt/carbon- it was clean.
and I just changed the oil

my tire pressure is always at 30 psi, hardly no A/C usage, RPMs kept below 3000 always,air filter is new, this is a full tank of city driving (small city, not alot of hills, mostly flat), I drive up to 50mph on some open roads thats it. So after all this I still get around 19-20 mpg.

The only thing left on my list is the spark plugs which I think could be making my car do all this.The parts guy at the GM dealer suggested regapping and cleaning the plugs carefully so they would last longer and run better. Besides that I'm assuming one of the sensors is slow (O2,MAP,Knock,TP,ECT) or there is a problem with the fuel system and i'm thinking about taking the car in for a check, Bad Fuel Economy Diagnositc its all in the service manaul, i just don't have a Tech 2.