When is the best time to fill up a gas tank? When it's half full. A gas tank that is half full has less air inside the tank, so it helps minimize the evaporation of gasoline when you're filling up. Less evaporation means more actual gas will stay inside the tank, giving you more fuel for your money. Also, always be sure to fill it up to a full tank; it'll cut back on your visits, saving you money in the long run.