2000 Oldsmobile Alero Excesive fuel usage

There could be one of several things causing you to guzzle gas, even a combination of things.

First, make sure your fuel gauge is working properly. Now here's a list of things to check:

Coolant temp sensor (may be causing you computer to stay in "Open-Loop")
Leaking fuel pressure regulator/injectors
Old O2 sensors
MAF/MAP sensors or the computer itself.

And finally, is your transmission shifting out of first? And if you are getting any check engine lights, have your computer scanned.