Junkno,
Rest assured that you really aren't getting 40 MPG. The fuel gauge is not an accurate way to calculate fuel mileage. It is not linear. The only time it is really accurate is when you have a full tank and when you are dead empty. All other times it is relative.
The only accurate way to calculate mileage is to record the miles traveled for a full fillup. Divide the miles by gallons and there is your true mileage for that tank. After many tanks you will get a true "average" MPG figure.
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