I was wondering how you know how far to drive before you stop and put exactly 1 gallon of gas in each time? Do you use a Scangauge and some kind of digital fuel level meter? It just seems that it would be hard to fill the tank to the same place each time and such a short distance traveled would seem to leave alot of room for error. More distance traveled and the resulting increase of fuel consumed would allow you to average out any inaccuracies.
"If the engine ain't running it ain't using gas!"
I bet what he's doing is saving the hassle of entering his full details. He probably calculates his MPG on his fuel receipt, then enters that calculation and "1" gallon. I don't think it's that he's adding only a gallon each time.
I usually keep a small notebook in the car with odometer and gallons used. I figure out my mileage when I fill up but by the time I get back to a computer I dont remember the miles and gallons so I just put the mpg in.
It almost always is around 8 gallons filling up so it doesn't really change the average. I posted a csv around here somewhere of the entire record of miles and gallons used but I can't find it. One of these days I will go back and edit the entries but I never seem to get around to it.