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Email Debunking - Tips on Pumping Gas

Gas PumpThere's an e-mail that I seem to get just about every time gas prices go up. So, with the recent jump in prices, it's found its way to me again. The first time I got it several years ago, Snopes hadn't yet addressed it, so I wrote a quick reply to the person who sent it to me. Snopes has addressed it by now, but I still thought I'd share an updated version of what I wrote originally. It's a little more concise than the Snopes article.

First, for reference, here's the e-mail in question*. Note the claim to authority at the start, and the request to pass it on at the end. What chain mail would be complete without them?


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?

The first time I read through this e-mail, I thought that it all seemed technically true, just not very significant. And the more I think about it, the more insignificant the effects seem.

The part about a gallon not being a gallon is silly wording, but I realize what the author was trying to get at - that the warmer it is, the less dense the gas is, so the less of it you're getting by mass. However, the fact that the tanks are buried underground means that temperature stays a lot more constant than if they were above ground, and any temperature fluctuation is pretty insignificant. In fact, I found this animated graph showing soil temperature by depth, and how it varies from hour to hour. One meter deep, and you can't see the graph moving at all**.

The part about gas evaporating if your tank's almost empty seems pretty silly, too. First of all, unless your tank was completely empty (like newly installed, never had a drop of gas in it - not likely if you actually drove your car to the station), your tank's already full of gasoline vapor. It's probably already at its max partial pressure. More gasoline won't evaporate unless you get rid of that vapor. But, even assuming that your tank was completely open to the atmosphere, gasoline doesn't evaporate that fast. I can think of plenty of projects where I've poured gas into a coffee can for cleaning a part or something, and I don't see the gas evaporating in front of my eyes. It usually doesn't take me more than a few minutes to fill up my tank, so I can't imagine that a significant amount of gas would evaporate in that time.

I wonder about the part on pumping gas faster making it evaporate more, too, for the same problem as above. Your gasoline may be slightly warmer because there's more work being done to it to get it to pump faster, but you've still got the problem that you're pumping it into a mostly closed container that's already going to be full of gasoline vapor. Plus, it's not like gas stations have terribly fast pumps - the pump at the airport I used to work at would put out a gallon every couple seconds or so (it was many years ago that I worked there, so I don't remember exactly).

The part about the tanker stirring up sediment seems like the best advice from the e-mail. However, according to that Snopes article, there are already filters in place at the gas station to minimize particulates making it into your tank.

Probably the best way to get the most mileage for your money is to adjust your driving habits, and not accelerate hard. I'd bet that would make your gas go a lot farther than only filling up on the slow setting on cold days.

* Some versions of the e-mail I've received were combined with another chain e-mail that had erroneous information about U.S. oil imports from the Middle East, and which companies were supposedly importing the most. Snopes has covered that one, as well.

** Also, with the thermal inertia of the fuel and tank, and the insulating properties of the tank, the fuel temperature will lag the ground temperature, meaning it's not at its coolest early in the morning. Assume that for a week, you have several days of the exact same weather. The ground temperature will oscillate from its high to low. The fuel will respond by being heated or cooled by the ground, but won't track it exactly. It will lag due to any insulating properties of the tanks. As long as the ground is warmer than the fuel, the fuel temperature will increase. When the ground temperature starts dropping, it's still a little warmer than the fuel, so the fuel temperature will continue increasing until the ground temperature finally drops below the fuel temperature. Now, the fuel will begin cooling, but lagging the ground temperature. When the ground temperature gets to its low point and starts increasing again, it will still be cooler than the fuel, so the fuel will continue to cool until the ground has warmed up some more. Still, with as little that the ground temperature varys, this is all academic, anyway.

Edited 2011-03-22 Moved the sentences about the start and end of the e-mail to before the blockquote, to improve flow.


Fuel storage tanks used in gas stations don't have IRF's (internal floating roofs). THose are only used on tanks that hold 75 cu meters (almost 20,000 gal.) or more of fuel. To imagine how much fuel that is, tanker trucks hold 3-5000 gallons of fuel.

Secondly, underground tanks used at gas stations MUST be double-walled, and insulated, therefore any above ground temp changes have no effect on the fuel density/temperature ratios.

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