2005 Cadillac CTS using to much gas

When was the last time you performed a tune up? And is there any drivability problems, such as rough idle, hesitations, stalls, black/white smoke from the tail pipe?

There are several possible causes to poor gas mileage. Some of them include: (Most common)
An engine in poor state of tune.
Faulty coolant temp sensor
Bad/old O2 sensors
Dirty air filter
Faulty/aging ignition system
leaking fuel pressure regulator