Paul,
In your fourth post, if I understand you correctly, you are basing your conclusion that you have poor milage on the one fill-up where you took 9+ gallons on thirty something miles. I would keep checking.
Perhaps during the previous fill-up you didn't really fill it to the top, so on the one fill-up where you did your calculation the tank took the 9 gallons. Keep checking and let us know. My guess is that you are really getting double digit mileage.
