1999 Saturn SL2 Bad Gas Mileage 12 miles per gallon

My car is getting 12 mpg both highway and city average. I don't hot rod it, and I warm it up in the winter before I bring it out. This mileage is consistent for both the winter and summer. I calculate my mileage by miles driven/gallons used.

I have no clue what is wrong. I brought it to the GM dealer and they said the car runs perfect.

Here is a list of everything I replaced:

Battery
Air Filter
EGR Valve
PCV Valve
NGK Plugs and Wires
Control Module
Starter
WIX #33731 Fuel Filter/Regulator
ECTS and Connector
AIT Sensor and Connector
MAP Sensor
Front Manifold Oxygen Sensor, OEM
Thermostat
Valve Cover Gasket
Cleaned the Throttle Body