Fill Up in the Morning
Not many people know this, but gas is a liquid that expands and contracts, depending on its temperature. By filling up your tank in the coolest part of the day, you will actually be getting more gasoline for your money, as it will have contracted overnight.
Buy on Tuesday or Wednesday
Gas prices are elevated in the latter part of the week in anticipation of the weekend rush. As such, you should buy your gas earlier in the week, but not too early. Weekend prices still linger at many stations on Monday morning.