If you want to get the most gas for your money, buy your gas early in the morning. Gas expands when it gets hot and this results in a gallon of hot gas having less energy in it than a gallon of cold gas (because there are fewer gas molecules in the hot gas). If you buy your gas in the morning, the gas will be at its coolest and densest so you'll get the most energy for your dollar. The difference is only a couple of percent, but it adds up over time.