There are 2 known issues that cause the Taurus to become gasaholics. The first thing to do is to have the OBD scanned for codes. The first issue is the EGR sensor that fails, sets a code, but does not trip the MIL light. The second is to remove the MAS ( mass air flow sensor) from the air snorkel and manually clean it with a can of MAS cleaner. By the way, make sure you are running on original Ford plugs or NGk. Ford engines hate Champions.
Avg mpg rating for this vehicle is 17 city, 20 hi-way. What is your's getting? Lot of difference in the gas from one tank full to the next also! Because it's a lot of difference in it from one truck load to the next! Fact!!