This is an article I found in the net. This is a very useful info to share...
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 vapor, 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'!