Tips on Pumping Gas

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Viral message purports to share a petroleum industry insider's tips for saving money at the gas pump. Do they really work?

Description: Viral message
Circulating since: Aug. 2007
Status: Mixed (details below)

Email contributed by Skip M., Aug. 24, 2007:

Gas Tips
I've been in petroleum pipeline business for about 31 years, currently working for the Kinder-Morgan Pipeline here in San Jose, CA. We deliver about 4 million gallons in a 24-hour period from the pipe line; one day it's diesel, the next day it's jet fuel and gasoline. We have 34 storage tanks here with a total capacity of 16,800,000 gallons. Here are some tricks to help you get your money's worth.
1. Fill up your car or truck in the morning when the temperature is still cool. Remember that all service stations have their storage tanks buried below ground; and the colder the ground, the denser the gasoline. When it gets warmer gasoline expands, so if you're filling up in the afternoon or in the evening, what should be a gallon is not exactly a gallon. In the petroleum business, the specific gravity and temperature of the fuel (gasoline, diesel, jet fuel, ethanol and other petroleum products) are significant. Every truckload that we load is temperature-compensated so that the indicated gallonage is actually the amount pumped. A one-degree rise in temperature is a big deal for businesses, but service stations don't have temperature compensation at their pumps.
2. If a tanker truck is filling the station's tank at the time you want to buy gas, do not fill up; most likely dirt and sludge in the tank is being stirred up when gas is being delivered, and you might be transferring that dirt from the bottom of their tank into your car's tank.
3. Fill up when your gas tank is half-full (or half-empty), because the more gas you have in your tank the less air there is and gasoline evaporates rapidly, especially when it's warm. (Gasoline storage tanks have an internal floating 'roof' membrane to act as a barrier between the gas and the atmosphere, thereby minimizing evaporation.)
4. If you look at the trigger you'll see that it has three delivery settings: slow, medium and high. When you're filling up do not squeeze the trigger of the nozzle to the high setting. You should be pumping at the slow setting, thereby minimizing vapors created while you are pumping. Hoses at the pump are corrugated; the corrugations act as a return path for vapor recovery from gas that already has been metered. If you are pumping at the high setting, the agitated gasoline contains more vapor, which is being sucked back into the underground tank so you're getting less gas for your money.
Hope this will help ease your 'pain at the pump'.

Analysis: As I researched the contents of this much-discussed viral text, I found disagreement among presumed experts as to the accuracy of the specific claims, but a general consensus that whatever modest savings might result from following these measures, they're probably more trouble than they're worth.

Let's take them one by one:

1. Fill your tank in the morning when the temperature is cooler so you get more volume for your money?

Yes and no. The basic science behind this is correct. Liquids expand as they warm. The figure usually cited for gasoline is about a 1 percent increase in volume per 15-degree rise in temperature. Therefore, if you buy 20 gallons of gas at a 90-degree temperature, due to expansion you end up with about 2 percent less product for your money than you would have gotten had you pumped 60-degree gasoline. At a retail price of $3.00 per gallon that differential would cost you $1.20.

The thing is, given that the gasoline is pumped from huge underground tanks wherein the temperature is less variable than that of the outside air, it's very unlikely you would encounter a 30-degree variance in fuel temperature in a 24-hour period. In fact, says a physicist interviewed by KLTV News in Jacksonville, over the course of a day the fuel temperature probably varies no more than a scant few degrees, so the actual savings from pumping in the morning would likely amount to only a few cents per fill-up.

2. Don't pump gas if a tanker truck is filling the station's holding tanks, because you'll end up putting dislodged sediment into your own tank?

Probably not. Modern gasoline holding tanks and pumping systems contain filters designed to block any such debris from reaching your car's gas tank. Should some particles squeak by, your engine's fuel filter ought to have no problem taking care of them.

3. Pump gas when your tank is no more than half-empty, because the emptier the tank the more you will lose to evaporation?

Yes and no. The idea here seems to be that the more unfilled space there is in the tank the more gasoline will be able to evaporate and escape into the atmosphere when you open the cap. Which makes sense, although according to physicist Ted Forringer the actual amount of vapor lost this way would be minuscule, adding up to only a few cents' worth per fill-up. A more important concern is the quality and fit of your gas cap, the job of which, in part, is to minimize evaporation on an ongoing basis. By one estimate, a poorly sealed gas cap can result in the evaporation of a gallon of gas in just two weeks' time.

4. Pump gas at the low-speed rather than the high-speed setting because the latter causes more agitation, thus more evaporation?

Probably not. It does seem logical to assume that the higher the speed of the pump the more it may agitate the fuel, causing more evaporation. But consider this: the longer it takes to pump the fuel the more evaporation can occur too, so any benefits to pumping at the slower speed are probably negated.

Gas Tips That Really Work

If all this just leaves you feeling frustrated and confused, don't despair. has actually tested some of the most common gas-saving tips and shares the ones that really work here and here.

Drive safely!

Sources and Further Reading

Saving on Gas: Fact or Fiction?
KLTV News, 4 April 2008

No Easy Way to Save Money (or the Earth) at Pump
Star-Ledger, 22 April 2008

Searching for Savings as Gas Prices Rise
Tallahassee Democrat, 12 April 2008

Are You Getting Ripped Off by 'Hot Gas'?
ABC News, 9 April 2007