Here's how to increase you MPG: by keeping the RPMs down.
An easy way to keep the RPMs down is to drive the speed limit. Especially if you live in a 55 MPH zone.
To experiment with this, I drove my Honda Civic 60 MPH back and worth to work, and I had to refill the tank once it reached 300 miles.
I then drove 55 MPH on the next tank. You know the results?
I was able to stretch the miles to 400 miles, increasing the per tank use by 33%!