I had a similar problem entering my first fill up after consuming the fuel the dealer topped off. I wanted to log the consumption of the first tank which Fuelly isn't set up to do well. I entered $0.10 as the lowest price per gallon, since it was actually $0.00. My average cost is off a little, but all the other stats work out well. I knew how many miles were driven the first tank so I counted from 0 to 536, my odometer reading at the time of first fill up.
The trouble with newly purchased cars & their first tank that someone else filled is that they generally only fill it so that it appears full on the dial, not full-full (the extra fuel is simply an expense to them). So the mpg from the first tank always tends to be a bit erratic. It only really settles down on the 2nd tank. It helps to top off the tank yourself & start from there.