The following is the crux of an interesting discussion we found on a recent forum post; whether you believe it or not is up to you!
DO NOT DONATE FUEL
Recently I came across a very useful tip. I was surprised to know it but had a doubt so I talked to one of the pump technicians and he too accepted it as a fact. I think apart from providing space for the gas generated inside the petrol tank this is yet another reason why we shouldn’t fill the tank to the brim. Many of us are not aware that the petrol kiosk pump has a return pipe-line (in Pink). When the petrol tank (in the car) reaches full level, there is a mechanism to trigger off the pump latch and at the same time a return-valve is opened (at the top of the pump station) to allow excess petrol to flow back into the pump. But the return petrol has already pass through the meter, meaning you are donating the petrol back to the Oil Dealer.
Also only 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 liter is not exactly a liter. 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.
Source : http://www.turbosociety.com/in-the-news/dont-donate-your-cash-to-gas-companies-some-important-tips