Tips for filling up.
First and foremost, if there is a delivery truck filling the underground storage tanks when you stop to buy petrol, DO NOT fill up! The pumping action will be stirring up all the crud at the bottom of the storage tanks and if you fill up now, despite the pump filters, some of that crud will end up in your tank. Come back in a couple of hours when the residue will have had time to settle again, or go to a different station.
Less is More.
Take advantage of early morning cold by filling up your tank early in the day, while the ground temperature is still cold. The temperature of gasoline, diesel, ethanol and other petroleum products plays an important role in getting the most for your money. Petrol stations do not have temperature compensation at the pumps (though loading and storage facilities do, ensuring them greater accuracy). Because the storage tanks are underground, the colder the ground the more dense the petrol. When it gets warmer it expands, so buying in the afternoon or in the evening your gallon is not exactly a gallon. Petrol pumps don't measure actual volume or density, they assume volume derived from flow-rate. The flow rate doesn't change with temperature but the density of the petrol does. So if the pump is calibrated to deliver an assumed gallon in 10 seconds based on full flow, if the petrol is colder, it will be more dense and you'll get marginally more in that 10 seconds than if you fill up in the afternoon.
Petrol evaporates more quickly than you might think.
When you're filling up, if you have time on your hands, fill up at about 1/3 of full power. This minimises the backflow of evaporating vapour created whilst you're filling up. The vapour has a chance to re-condense in your tank rather than being wasted into the outside air (or if you're in America, sucked back into the underground tank via the vapour recovery collar on the pump).
Fill up when your gas tank is half empty.
The first reason is similar to the top tip - crud in the bottom of the tank. If you let your tank get nearly empty, your fuel pump will start pulling the crud in the bottom of your tank and you run more risk of blocking a fuel filter. The other reason though is that the more gas you have in your tank, the less air there is above it to be filled with vapour that will escape as soon as you take the petrol cap off.
Now how much of a difference will these sorts of things make in the real world? One a single tank, not much. Over the course of a year, you might actually notice a difference. Over the lifetime of your vehicle, probably a measurable difference.
