Your method for sampling mileage is not very reliable. Different pumps will stop at different points, and with only putting in 4 gallons, a quarter gallon one side or the other could make a pretty big difference.
I track every tank, and fill when it gets low - usually before the warning. I try to use the same pump, but that's not always possible.
So far, with 5894 miles of ownership on my 2002 3.2
Average: 21.41mpg
Best tank: 24.08mpg
Worst tank: 17.71mpg
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