I got this as a mail forward and thought of sharing it (Few things may apply only to countries like India)
1. Only buy or fill up your vehicle 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 fuel, when it gets warmer petrol expands, so buying in the afternoon or in the evening....your litere 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.
2. 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. In slow mode you should be pumping on low speed, 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.
3. One of the most important tips is to fill up when your tank is HALF FULL. The reason for this is, the more fuel you have in your tank the less air occupying its empty space. petrol evaporates faster than you can imagine. Petroleum 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.
4. Another reminder, if there is a fuel truck pumping into the storage tanks when you stop to buy, DO NOT fill up--most likely the petrol/diesel is being stirred up as the fuel is being delivered, and you might pick up some of the dirt that normally settles on the bottom.
Hope this will help you get the most value for your money.