some facts to be aware of when you are pumping gas...

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.
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.
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.)
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'.