Poor gas mileage

I have a 2004 Avalon XLS with 16,500 miles. I have yet to crack 20 MPG even on long trips. I'm no ricky-racer. Tire pressure is up to snuff, idles smooth & drives like a dream. I'm no longer getting the quick engine start-up. Typically takes 2 seconds or so for the engine to fire up whereas when new, it jumped to life almost instantly.

Any suggestions?

You should be able to do better than that for mileage! Is the car still under warrenty? The dealer should look into this.

How's the air filter look?