Since November when I moved to AZ. I have been getting poor mileage. Predominantely I have been getting regular gas at Sam's club & I have been getting 8-9 MPG. Last Sunday Sam's was closed so I went to Mobil I have driven 50 miles since Sunday and the gauge is at the top line of the F (down from about 1/16th above where it lands when full). On the 8-9 MPG tanks I would be near the 3/4 point after 50 miles. Does it make sense that Sam's gas would perform this poorly?