If you ever find yourself stopping at a convenience store to pick up some milk or a cup of coffee and notice your gas tank is half full, consider filling up. When your gas tank is completely empty, there's extra air inside. Gasoline evaporates quickly -- and even more so in the warmer spring and summer months. When there is more gas in the tank than air, there's less evaporation, and you get more gas for your money. But a tip: Don't make trips just to fill up when it's half full. The added time to drive and fill negates the savings.