I got the following from a friend - hope it helps!
Here at the Shell Pipeline where I work, we deliver about 4 million litres in a 24-hour period. One day is diesel the next day is jet fuel, and petrol, regular and premium grades. We have 34-storage tanks here with a total capacity of 16,800,000 Litres.  
Only buy or fill up your car or truck in the early morning when the ground temperature is still cold. Remember that all service stations have their storage tanks buried below ground. The colder the ground  the more dense the petrol, when it gets warmer  petrol expands, so buying in the afternoon or in  the evening....your litre is not exactly a  litre. In the petroleum business, the specific gravity and the temperature of the petrol, diesel and jet fuel, ethanol and other petroleum products plays an important  role.  
 
A  1-degree rise in temperature is a big deal for this business. But the service stations do not have temperature compensation at the pumps.  
When you're filling up do not squeeze the trigger of the nozzle to a fast mode  If you look you will see that the trigger has  three (3) stages: low, middle, and high. You should be pumping on low mode, thereby minimizing the vapours that are created while  you are pumping. All hoses at the pump have a vapour return. If you are pumping on the fast rate, some of the liquid that goes to your tank becomes vapour. Those vapours are being sucked up and back into the underground storage tank so you're getting less worth for your money.  
 
One of the most important tips is to fill up when  your Petrol tank is HALF FULL. The reason  for this is the more Petrol you have in your tank the less air occupying its empty space.  petrol evaporates faster than you can imagine.  petrol storage tanks have an internal floating  roof. This roof serves as zero clearance between  the Petrol and the atmosphere, so it minimizes the evaporation. Unlike service stations, here  where I work, every truck that we load is  temperature compensated so that every litre is  actually the exact amount.
Another  reminder, if there is a petrol truck pumping  into the storage tanks when you stop to buy  Petrol, DO NOT fill up; most likely  the petrol is being stirred up as the Petrol is  being delivered, and you might pick up some of  the dirt that normally settles on the  bottom.