It’s simple physics. When the temperature of gasoline rises above 60 degrees, gasoline expands. The amount of energy the gallon delivers shrinks by volume. Since our gas pumps don’t account for the bigger volume, motorists receive less gasoline than they should. Here’s the whole story, from a prize-winning series by the Kansas City Star:
Hot Fuel For You, Cold Cash For Big Oil
Highway Funding Shortchanged, Loophole Enhances "Hot Fuel" Profits
Hot Fuel Triggers Investigation