Did you know Monday and Tuesday are the best days of the week to fill up on gas? Statistically, gas prices rise by Wednesday as demand is calculated and prices are changed to reflect it, so get to the pumps at the beginning of the week. While you're at it, don't fill up when you're nearly out of gas. A gas tank that is half-full has less air inside the tank, so it helps minimize evaporation as you're filling up. Less evaporation means more fuel staying inside the tank, so you get the real value of the gas that you're paying for.