There is a chain post making the rounds in my social networks. It asserts that gasoline vapors can affect how much fuel you actually get. To minimize the amount of vapor passed through the nozzle/meter you should:

  • Pump gas at the slow setting
  • Pump gas when the cars tank is half full or more
  • Not pump gas when the delivery truck is refilling the big station tanks

The post reeks of a chain letter and unverified science.

Is there any truth to the premise that gasoline vapors impact the metering of fuel? In other words will a gas pump measure liquid fuel volume or any fluid fuel volume?

If the premise is true, do the 3 behaviors actually reduce fuel vapors?

How much does gasoline expand and contract when temperatures change?

TIPS ON PUMPING GAS I don't know what you guys are paying for gasoline.... but here in California we are paying up to $3.75 to $4.10 per gallon.

My line of work is in petroleum for about 31 years now, so here are some tricks to get more of your money's worth for every gallon:

Here at the Kinder Morgan Pipeline where I work in San Jose , CA we deliver about 4 million gallons in a 24-hour period thru the pipeline.. One day is diesel the next day is jet fuel, and gasoline, regular and premium grades. We have 34-storage tanks here with a total capacity of 16,800,000 gallons.

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 gasoline, when it gets warmer gasoline expands, so buying in the afternoon or in the evening....your gallon is not exactly a gallon.

In the petroleum business, the specific gravity and the temperature of the gasoline, 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 vapors that are created while you are pumping. All hoses at the pump have a vapor return. If you are pumping on the fast rate, some of the liquid that goes to your tank becomes vapor. Those vapors 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 gas tank is HALF FULL. The reason for this is the more gas you have in your tank the less air occupying its empty space. Gasoline evaporates faster than you can imagine. Gasoline storage tanks have an internal floating roof. This roof serves as zero clearance between the gas 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 gallon is actually the exact amount.

Another reminder, if there is a gasoline truck pumping into the storage tanks when you stop to buy gas, DO NOT fill up; most likely the gasoline is being stirred up as the gas is being delivered, and you might pick up some of the dirt that normally settles on the bottom.

To have an impact, we need to reach literally millions of gas buyers. It's really simple to do. I'm sending this note to about thirty people. If each of you send it to at least ten more (30 x 10 = 300)...and those 300 send it to at least ten more (300 x 10 = 3,000) and so on, by the time the message reaches the sixth generation of people, we will have reached over THREE MILLION consumers !!!!!!! If those three million get excited and pass this on to ten friends each, then 30 million people will have been contacted! If It goes one level further, you guessed it..... THREE HUNDRED MILLION PEOPLE!!! Again, all you have to do is send this to 10 people. How long would it take?

  • 1
    possible duplicate of Does the time of day affect fuel economy? Mar 20, 2013 at 18:30
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    While related, I don't see this as a duplicate. The claims are distinct. Notice that temperature is not part of any of these claims and is integral to the other. Mar 20, 2013 at 18:37
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    The possible duplicate leads to a Snopes article which disputes some of the vapor claims. Not sure if that makes it a dupe or not. snopes.com/inboxer/household/gastips.asp
    – Freiheit
    Mar 20, 2013 at 18:57
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    While we are debunking claims, the last part of the email about THREE HUNDRED MILLION PEOPLE is also false because there will be overlap in the recipients. Mar 21, 2013 at 16:16
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    I never trust anything written by someone who doesn't know how to use paragraphs Mar 21, 2013 at 23:58

3 Answers 3


Verbatim debunked here: http://urbanlegends.about.com/od/automobiles/a/pumping_gas.htm

The physical view:

Slow tanking claim and half full tanking claim have the same fallacy:

While driving, the fuel/air system in your tank will be thorougly agitated from the vibrations of driving. We can assume the air inside the tank is fully saturated. Vapor pressure of gasoline at 20 °C is about 60 kPa - this means the air contains up to 6% volume of gasoline.

No matter on what setting you fill up, what escapes is fully saturated air - it cannot take more fumes with it than it´s limit.

One might argue that the fuel we are tanking emits pure fumes, but it doesnt. It does not emit fumes if it is in a saturated environment, that´s the magic of vapor pressure.

read http://en.wikipedia.org/wiki/Fuel_dispenser#The_metrology_of_gasoline

Now from yachts I know that there are bacteria that thrive in diesel tanks. See http://www.hpcdfuel.com/pdf/DOWfuel_training.pdf, but it takes a serious storm to shake it loose. I would not assume filling up a tank shakes the sediment loose. The tanker trucks get cleaned regularly, so they cannot bring sediment.

  • 60 kPa is 60% of 101.3 kPa = 1013 hPa ... Mar 20, 2013 at 22:45
  • While sediment is not a big issue, water can be. Ethanol, such as in E10 fuels, is hygroscopic; it pulls water from wherever it can get it, including the air. Ethanol will mix with gasoline and with water, but it isn't an emulsifier; the water, with some of the ethanol, will come out of solution and separate in a layer on the bottom. If a station hasn't purged its storage tanks in a while, and you fill up when the tanker's filling or when the tank is very low, you can pull water into your tank, which is more acutely damaging than sediment (most of which will be caught by the fuel filter).
    – KeithS
    Mar 21, 2013 at 16:55

Remember that all service stations have their storage tanks buried below ground.

... which means they have approximately constant temperature over the day (see table in Georgios Florides and Soteris Kalogirou: Measurements of Ground Temperature at Various Depths).

As for the economic loss by due to the vapor:

The strategy of tanking half full doesn't save you anything - it is true that you'd loose only half as much during each tanking as only half of tank's volume of saturated gasoline vapor is pushed out by the inflowing gasoline. But you need to tank twice as often.

How much do you actually loose?

A general rule of thumb is that evaporation of a liquid will increase the volume by a factor of 1000 (calculate more exactly from density of liquid, molar mass, temperature/partial pressure).
So assuming an empty 60 l tank still saturated with gasoline vapor. The DVPE is the vapor pressure at 38°C, which is specified in norms. E.g. for "summer blends" of gasoline, it can be 60 kPa. Normal pressure is 101.325 kPa. So at +38°C air temperature, roughly 36 ml of gasoline would be in the vapor.
This is the equivalent of driving something between 250 and 1200m.
Todays gas price here in Germany is 1,529 €/l, so the loss would be 5.5 €ct (if it was +38°C instead of snowing...). You could also save 5.5 €ct by getting 30 l of gasoline for 0.2 €ct/l less, i.e. buying gasoline if it is about 0.1% cheaper. I'm calculating this on 30 l, because if you manage to get a full tank of gasoline for 1 €ct/l less by not tanking when the tank is still half full, you'd save 60 €ct, i.e. 10 times as much (FYI daily differences in gas price can easily be 5 - 7 €ct/l here).

  • true for most places, but I have seen in some countries and areas above ground storage tanks at gas stations. Mostly these are in scarcely populated mountainous terrain with very rocky ground in poor countries where it'd be just too expensive to bury the things.
    – jwenting
    Mar 21, 2013 at 7:04
  • @jwenting: sure. I have seen overground only for small sizes, like on a farm to tank just their machines. Anyways, the underground storage is actually part of the claim the OP cited. Mar 21, 2013 at 8:45

In the car

I think the major issue here is vapor pressure. Your gas tank is relatively air tight, ie vapors from the gasoline typically won't escape. We cannot assume the vapor pressure is such that no vapor escapes from the fluid since I have never heard of a air tight, pressurized fuel tank. Fuel vapor might escape when refueling; newer gas pump nozels with vapor arrestors mitigate this problem so we might assume gas vapor loss is negligible.

In the storage tank

These also are not pressurized storage tanks. However, they do have filters (sludge is negligible) and are air tight due to EPA regulations as in they must be resolved against leaks when sealed and all leaks greater than 0.01 gal/min must be detectable. In an air tight tank, the vapor pressure is equal, therefore the gas cannot evaporate as the air in the tank keeps the gas in liquid form. Pumps draw from the bottom of the tank, so we can assume 100% of the fluid volume pumped is liquid volume. However, due to some specific gravity and temperature tricks as mentioned, it could could be less dense or contain more water.

I would say, with confidence, this is a complete myth.

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    "Your gas tank is relatively air tight, ie vapors from the gasoline typically won't escape. ...since I have never heard of a air tight, pressurized fuel tank." - these two sentences are in direct contradiction to each other. The correct one is that the tank has a connection to the outside, so neither overpressure (hot weather) or underpressure (fuel is consumed by the engine) occurs. Mar 20, 2013 at 22:08
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    Welcome to Skeptics! Please provide some references to support your claims. Mar 21, 2013 at 2:02
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    only fuels stored in pressurised condition in cars are LNG, LPG, and hydrogen, not gasoline. And those are pumped in through pressurised hoses attached through pressure sealed valves at both ends. They're DESIGNED to prevent the escape of the fuel gasses/vapours.
    – jwenting
    Mar 21, 2013 at 7:06
  • @cbeleites I believe you are mistaken. If a gas tank is not air right, yet relatively so, such that vapors rarely escape and only under assumed/predicted conditions (vehicle operation and refueling) then a truly airtight tank does not exist for vehicles (except for those mentioned a la @jwenting). I believe I should have clarified my answer to include vapor escape during operation and refueling are assumed to be normal yet vapor escape otherwise is not.
    – endowdly
    Mar 25, 2013 at 20:31

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