That seems way too low for in town driving, even for a 3.1.  Manual or
automatic transmission?  If auto, is it shifting correctly?  What was
the original in town and highway rating for that car?

"The car runs well" but there must be some explanation for that much
fuel loss.  Failure in mixture control or ignition timing?  Bad cylinder
but you just didn't notice because it's a big engine?  Leak in gas line
or gas tank?  Any unexplained gasoline odors lately?  Theft?

Conservation of mass applies.  That gas is going somewhere, either out
the tail pipe, onto the ground, or into somebody's siphon hose.

If it's the engine, then a good diagnostic is probably in order.
Timing, compression, emissions check, diag codes, etc.  I'm just
throwing ideas here, sorry I can't be of more specific help.

--Dave

Don't know where you live, but has the weather been very cold there
lately?

Do you use the handbrake in the winter?  Do not use handbrake in
winter if you live in a climate where the roads get snow covered.

Someone has already asked about whether it is stick or auto.  You can
do a lot with a stick to improve mileage.

That is low even for burned out O2 sensors.  As another poster said,
leak, siphon,  or out the tailpipe,
More than likely the catcon is burned out, too.
Check all intake gaskets and hoses for vac / air leaks.
Check fuel pressure, regulator, and maf.
Check O2 sensors.

Thanks for the replies so far.  I have a couple of questions.  The car has 2
oxygen sensors.  Which one should I replace?  And, I am not getting any
"check engine light".  Shouldn't the light be on if the sensors are
defective?
More on the car, it has an auto trans and I am sure there are no fuel leaks
because I had the fuel line replaced back in Oct.  I have owned 2 of these
cars in the past and the gas mileage was always poor during the Winter
around 14 when it is below 20, but in the Summer it should get around 19.  I
only drive about 200 miles per month, so an inexpensive repair would be
best.  With the price of gas hovering around $3.19 in our area, it would be
nice to get better gas mileage.  Also, I don't know if it is running rich
because there are no emission tests in our area.
Marty