[TKWPrime]

Always remember that gas station pumps are rarely spot on accurate. All jurisdictions allow some percentage error in measurement. Some allow as much as +/- 5%.

Let's say you start with a full tank and drive 300 miles. When you fill up the pump registered 10 gallons. However, that pump reads 2.5% low so you really put 10.25 gallons (the real amount you burned) in your tank. You figure your FE based on the pump readout and figure you got 30 mpg.

You then drive back to your starting point and fill up again. But, this pump reads 2.5% high. You put 10.25 gallons (the real amount you burned) into your tank but the pump readout is 10.4 gallons. You figure your mileage based on the pump readout and figure you got only 28.85 mpg.

If the first pump is Texaco and the second pump is Mobil, you arrive at the erroneous conclusion that you get better mileage burning Texaco when in reality you got exactly the same mileage from both.