dude, for 12mpg you MUST have a hole in your gas tank...

but check for:
- vacuum leaks
- spark plugs...which once did you put???
- 500lbs dead man in your trunk
.....
I used to get low mpg several weeks before I get a O2 code, so you might have that comming soon (for the one you haven't changed yet)..or like someone said, could be the IMRC...i had that problem too.