When Is A Gallon Not a Gallon? When You're Buying Gas

On a hot August day in Texas, you notice your big pickup is nearly running on fumes, so you drive into a gas station where the price is $2.69 a gallon. You fill up the tank with 35 gallons of gas. Simple math tells you that's going to cost you $94.15. You're not happy about it, but at least you know across the country people are paying something close to the same for gas, so you go inside and get ready to pay. You tell the clerk behind the counter what pump it was, and she says, "Okay, $94.15 plus the $1.14 surcharge, that's $95.29."

"Hey, what's this surcharge business?"

"That's the heat surcharge. Any time the gas is hotter than sixty degrees, we get to charge extra for the same amount of gas."

You think something unprintable, but you've been brought up to be polite to ladies, so all you say is, "Well, I'll pay it, but this is the last time I'm buying gas here."

"Doesn't matter. They all do it."

Sound crazy? Well, it isn't. It happens every day, all across the country. Only the surcharge is a hidden one, and perfectly legal, so the sales clerks don't talk about it. As an article in the Aug. 27 Kansas City Star describes, if you buy gas in the U. S. that is hotter than sixty degrees F—and as a recent U. S. government study shows, that is most of us in the warmer parts of the country—you don't get what you think you're paying for. Here's how it works.

Each molecule of gasoline provides a certain amount of energy to your car's engine. What you're really buying when you pay for gas is energy, and so you'd think that the fairest way to charge for gas is so much money for so many molecules. Well, counting molecules is not too easy, so long ago it was decided that gasoline would be sold by the gallon, since measuring volume is simple and accurate.

The only trouble with that is that gasoline expands when it gets warmer. That means the same number of molecules take up more room at higher temperatures. If you have thirty-five gallons of gas at a temperature of sixty degrees F and warm it up to eighty degrees F, it's still a liquid. But it expands to occupy a volume of 35.42 gallons. Same number of molecules, same amount of energy—but a larger volume. And if you come along and buy 35 gallons of warm gas when it's eighty degrees, you pay the same money, but you get less gas (fewer molecules, less energy) than if it was at sixty degrees.

Petroleum engineers knew all about this decades ago, so they agreed on a standard temperature of 60 F for measuring volumes. And in the bulk transactions among refiners, pipeline operators, and wholesalers, you can bet that they take this expansion into account. In Canada, where the gas that's sold is on average colder than 60 F, an enterprising inventor went around to gasoline retailers and pointed out that when they sold cold gas, they were giving their customers more than they legally had to. So now nearly every gas pump in Canada compensates for temperature and delivers slightly less gas than it used to, for the same price on the pump screen. And the retailers happily paid the inventor for his idea, since they now make more money on each gallon of cold gas.

Nobody that the Star reporter talked to would admit it, but the reason temperature-compensated gasoline sales haven't spread to the U. S. is probably because the retailers would lose money instead of making money.

Engineering ethics is about experts who use their specialized knowledge for the good of their employers and society in general. Here is a clear case where an obscure technical effect is taking billions of dollars a year away from consumers. Engineers know about it, but the law permits it. As long as a gas pump delivers one gallon with a certain accuracy, it gets the stamp of approval from the local weights and measures authority and there is nothing illegal going on. But all that is legal is not moral, and the example of Canada shows that the technical fix for the problem is available at a cost that would not drive gas stations out of business.

Is this the worst problem you'll run into today? If it is, you're having a pretty good day. But it's real, and as gas prices go even higher, any changes in a direction that would improve matters would be welcome. Sooner or later, some clever advertising executive may think of a way to turn temperature-compensated gas sales into a competitive advantage. Unfortunately, logic does not play a large role in advertising, and the campaign to get this idea across would have to be very well planned. Another alternative is to change the laws regulating retail gas sales to require all gas to be sold with temperature compensation. The effect of this would be to make prices slightly lower in the summer and higher in winter, which might do something to offset the annual rise caused by summer driving. The prospects of either happening are not good. But until retailers start selling gas with the price compensated for temperature, at least now you know what you're paying for—which is not what it seems to be.

Sources: The original Kansas City Star article is at http://www.kansascity.com/mld/kansascity/15370193.htm. A method of figuring out what temperature does to the volume of gasoline is available from the Ohio Department of Transportation at www.dot.state.oh.us/construction/oca/Specs/SSandPN2002/10600402for2002.pdf.