Xcel's point is good. If you record your mileage and record the gallons, the error caused by mis-fills, partial-fills, and over-fills goes away over the lifetime of the car.

 

Most pumps will print a receipt that shows the purchase in gallons to three decimal places,
or about 1/15 oz. You can do as many partial fills as you like if you keep track of what went in between fillups. The higher the total gallons, the smaller (percentage wise) the uncertainty
due to the variation in the volume in a "full" tank. I just did a 26 gal calculation on an insight (10.5 gallon tank) and the results agreed with the fcd within 0.2mpg.