[Funny]

It seems counter-intuitive that you would get better gas mileage on a full tank. As the fuel in the tank is being burned, the car is decreasing in weight (unless the occupant is eating a bunch of Big Macs.)

So it stands to reason that the car would get better economy while the tank was getting lighter and lighter. A gallon of gas weighs approximately 6 pounds (2.72kg), my tank holds about 13 gallons, and I usually fill up 10+ gallons at a time. If my math serves me correctly, this translates to a 60+ pound (27.2kg) swing in weight. Last time I checked, reducing the weight in the car increased economy.

I don't have any experience with repairing gas tanks or fuel pumps, but unless the pressure on the fuel pump at a full tank is significantly higher or lower than at half or lower, or the percent weight distribution has a major affect mileage, then there would be absolutely no reason to believe that you get better economy on a full than a partial.