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 gauged. And while you're at it, don’t wait to fill up when you’re nearly out of gas. A gas tank that is half full has less air inside, 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 you’re paying for.