Gasoline is volatile hence prone to vaporization. The smell from your tank, when first opened, is that of trapped and pressurized gasoline vapors escaping. Also when you pump, the splashes within the fuel tank encourages fumes to escape, which is why all pumps in gas stations in the United States have caps on them. Recently, I learned about a way to save gas, that I have not heard of, so perhaps AskPatty readers may also want to learn about it. To get the maximum bang for your buck at retail gas station, here are some tips I received from a forwarded email, which I have since checked against other sources online to verify its claims.
Firstly, pump when temperature is lower, like in the early part of the day or later at night. The reason being, when the ground temperature is cooler, the gasoline in the underground storage tanks is more condensed, hence you get more of what comes through the pipe. Remember physics? When it is colder the molecules within the vapor move less and are thus more densely packed together in the same volume. So you get more gasoline molecules per gallon! Gasoline expands 1% per Fahrenheit change. Diesel? 0.6%. However, beware that the underground storage tank is huge and more insulated than air, so the temperature does not change as much. This impact is more notable between summer and winter, but then you can't not use gas in the summer. Take this tip with a grain of salt.
Anyway, the science and politics behind it is more interesting. Temperature compensation is used by the petroleum industry when calculating production amount, and when filling up trucks, but NOT at the retail pump. A quick Google search reveals a rather active debate on this topic. It seems that temperature compensation at retail pumps happens in Canada (calibrated to 15C), and in Hawaii (80F), but such is not the case nation-wide. And if you live in the colder state, you benefit more. Download temperature_compensation_slides.pdf .
This means that depending on the temperature (and altitude too... I would guess), a gallon is not a gallon.
Secondly, set nozzle on low. Those little notches to rest the nozzle when you hands get tired of squeezing them come in three settings -- low, medium, and high. How hard you squeeze the nozzle also affects how much gasoline is vaporized. These vapors get sucked back through the pump via a parallel vapor return system that returns the gasoline vapor to the underground storage system. This system is needed to prevent air pollution and to protect consumer health. The number at the pump may still be running forward as it dispenses liquid gasoline, but the extra churn created by setting the nozzle on high returns vapor to the gas station storage that you don't get credit for! (Do you know that gasoline vapor contains VOC, or volatile organic compounds, such as benzene, toluene and 1,3-butadiene, that are harmful to your lungs and cause cancer in the long run? These compounds also contribute to ozone, smog, and brown haze of summers.)
Thirdly, fill up regularly, e.g. when your gas tank is around half full (or empty -- depending on what type of outlook you have :-P ). This reduces air space where vapor can form. Since gasoline is volatile, it evaporates faster than you can blink... the more air space, the most you lose when the gas cap is popped open. This problem is prevented in industrial storage tanks via the use of "floating roof", which rises and falls with the liquid level inside the tank. Floating roofs are considered a safety requirement as well as a pollution prevention measure for many industries including petroleum refining.
Fourthly, don't overfill.
When you try to overfill a little to be safe, you ain't getting more gas if your station is equipped with vapor recovery systems. This is because when the gas pump automatically shuts off, a vapor lock blocks more gas from entering your car. Excess gasoline will just be sucked back into the storage tank, even if the meter is still running forward.
This EPA website sums it up succintly as well as having nice graphics that illustrates it. Over-topping wastes money, pollutes the environment, and ensures the next person using the pump pollutes too.If all these steps sound complicated, you are not alone. But with gasoline prices closing in on $4 in California, it makes me willing to adopt some of these new behaviors. Plus, now that I know better, it is just hard to worry about the impact of these vapors to my health... I won't do this just to save money -- the amount saved is decent but quite negligible (estimated $30-$50 per gasoline car per year, or $400-700 per diesel truck per year) but I will definitely be more careful with these vapors for the sake of my health and for the health of those in my car.