I hit just over 400 miles once, when my car was about 2 years old. Now, I'm lucky if I get much over 300 miles to a tank.
Just wanted to quote this and re-emphasize the role that air pressure plays in fuel mileage. When you are going slower, there is less air resistance pushing against the car. As you increase speed, it has a similar affect to driving up a hill. Slow and steady is how those hypermilers got 1000 miles out of a tank of gas on the new Sonata.Lately I've been driving speed limit or speed limit+5 because I got a speeding ticket in July and my gas mileage has increased 25-30 miles per tank (so like an extra gallon worth). Good stuff.