Read this article for a description of how the PHEV industry cooperated to deal with this issue, taking a big step beyond all previous gasoline-fueled cars: How designing the Chevy Volt's sealed gas tank brought automakers, CARB together "... but now we've got the scoop on the gas tank used to keep liquid fuel in the Volt from evaporating or going stale for up to a year. " It isn't the first batch of air that is a meaningful problem. It is the continual exchange of fresh air into that space that creates the problem. PHEVs don't allow that continual exchange. A Prime's fuel tank holds about 11.3 gallons, or 1.5 cubic feet. If the tank is almost completely drained of fuel and is holding almost entirely air, that is about 1.8 ounces of air at sea level, which is enough to fully oxidize only 1/8th of an ounce of gasoline. That isn't very much ruined gasoline when diluted by several gallons of good stuff.