In a comment on this article fenria14 wrote:
Gas stations sell a gallon of gasoline based on standard weight at a temp of 60 degrees F. As the temp rises, gasoline starts to evaporate into it’s vapor form, so if you’re buying a gallon of gas when it’s 80 degrees F outside, you’re paying for a full gallon but are instead getting less than a gallon of gas and gas vapor.
. . . if you live anywhere it regularly gets over 60 degrees F, (most of the US), you’ll get more for your money buying your gas at night or in the early morning when it’s close to 60 degrees F.