good info found elsewhere, thought it might be useful...... this info comes from somebody that supposedly has been in petroleum pipeline business for a long time, and they deliver about 4 million gallons in a 24-hour period from the pipeline; one day it's diesel, the next day it's jet fuel and gasoline. They have 34 storage tanks here with a total capacity of 16,800,000 gallons. Here are some tricks to help you get your money's worth: 1. Fill up your car or truck in the morning when the temperature is still cool. Remember that all service stations have their storage tanks buried below ground; and the colder the ground, the denser the gasoline. When it gets warmer gasoline expands, so if you're filling up in the afternoon or in the evening, what should be a gallon is not exactly a gallon. In the petroleum business, the specific gravity and temperature of the fuel (gasoline, diesel, jet fuel, ethanol and other petroleum products) are significant. Every truckload that we load is temperature-compensated so that the indicated gallonage is actually the amount pumped. A one-degree rise in temperature is a big deal for businesses, but service stations don't have temperature compensation at their pumps. 2. If a tanker truck is filling the station's tank at the time you want to buy gas, do not fill up; most likely dirt and sludge in the tank is being stirred up when gas is being delivered, and you might be transferring that dirt from the bottom of their tank into your car's tank. 3. Fill up when your gas tank is half-full (or half-empty), because the more gas you have in your tank the less air there is and gasoline evaporates rapidly, especially when it's warm. (Gasoline storage tanks have an internal floating 'roof' membrane to act as a barrier between the gas and the atmosphere, thereby minimizing evaporation.) 4. If you look at the trigger you'll see that it has three delivery settings: slow, medium and high. When you're filling up do not squeeze the trigger of the nozzle to the high setting. You should be pumping at the slow setting, thereby minimizing vapors created while you are pumping. Hoses at the pump are corrugated; the corrugations act as a return path for vapor recovery from gas that already has been metered. If you are pumping at the high setting, the agitated gasoline contains more vapors, which is being sucked back into the underground tank, so you're getting less gas for your money. Hope this will help ease your 'pain at the pump. 5. Do not top off your gas tank, when the pump shuts off, do not keep trying to add more gas..................a friend who owns a gas station says that by doing this, you are actually giving the next customer a $1.00 worth of gas. The gas you pump stays in the hose and never makes it to your tank...........good to know.