16 MPG from my Chevy S10

I bought myself a used 1999 chevy S10 and I love everything about it
except it's sucking up gas like crazy.
It's an automatic with the 2.2 liter engine. I thought it would be
economical but it's not. I'm getting 300 miles or so from a tank (18
gals) The dealer had told me he gave it a recent tuneup. Should I try
to get this remedied or is this as good a it gets? What's the most
likely cause of poor gas mileage, O2 sensor? Thanks in advance.