2003 Chevy Monte Carlo fuel economy

Engine Performance problem
2003 Chevy Monte Carlo Front Wheel Drive Automatic 75,000 miles

I am getting about 350 miles per fill with my Monte. I have looked at a number of sites which indicate that I should be getting far better fuel efficiency, particularly given close to 90% of my driving is on the highway. I have no codes registering, the pcv valve is good, I replaced the air filter and I drive to use the least amount of fuel possible. When I first purchased the car I was getting about 450 miles per tank but it has gone down hill since. Any ideas would be appreciated.