Again, I'll point out, this calculation is flawed.

You put in a quantity of gasoline when you filled up the car, and you drove a certain number of miles on that quantity. You then refilled the car with another quantity, and used the mileage from the previous quantity and the volume of the current quantity to calculate the MPG.

A given quantity of gas can have different energy content depending on the manufacturer, temperature, and lots of other factors. You are only coming up with an estimate when you use that type of calculation, because you are assuming that each gallon of gas you pump has exactly the same energy content, which is impossible.

I

'd be willing to wager that the differences are more than minute. Unless you are using the same pump, at the same time of day from the same station, you are going to have variances in both density of gas per gallon, as well as energy content per gallon.