Found this on the internet, geek heaven

Pump my Ride:

1. Pump petrol 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 petrol. When it gets warmer petrol expands, so if you’re filling up in the afternoon or in the evening, what should be a liter is not exactly a liter. In the petroleum business, the specific gravity and temperature of the fuel are significant. A one-degree rise in temperature is a big deal 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 petrol, do not fill up. Most likely dirt and sludge in the tank is being stirred up when petrol 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 petrol you have in your tank the less air there is and petrol evaporates rapidly, especially when it’s warm. (petrol storage tanks have an internal floating ‘roof’ membrane to act as a barrier between the gas and the atmosphere, thereby minimizing evaporation.)

4. Don’t be trigger happy. 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 petrol contains more vapor, which is being sucked back into the underground tank, so you’re getting less petrol for your money.

i knew about point 4 but the rest ininteresting