“Miles per gallon” (mpg) is the most common measure of a car’s fuel efficiency. The typical U.S. consumer, in shopping for a car, uses mpg as a way of calculating gas consumption and carbon emissions.

Because the concept is in such wide use, mpg has become as familiar to the American ear as FBI, CIA, or ABC.

But miles per gallon is not the best way to measure how fuel-efficient one car is compared with another.

That’s according to Richard P. Larrick and Jack B. Soll, management professors at Duke University whose math-intensive argument, “The MPG Illusion,” appeared in the magazine Science last summer.

Larrick was at Harvard on April 9 to make the case for an alternative metric of automotive fuel efficiency — one that favors volume over distance: gallons per mile (gpm). He and Soll, in fact, like using this measure per 10,000 miles. (That’s the average number of miles Americans drive in a year.)

Car buyers wrongly assume that gas consumption measured in miles per gallon goes down in a straight line: The higher the mpg, the lower the gallons of gasoline burned.

But in reality, fuel efficiencies are curvilinear, said Larrick. The higher mpg ratings go up, after about 20 mpg, the more efficiencies flatten out.

Because of this misperception — Larrick called it “the mpg illusion” — people underestimate the value of improving a gas guzzler’s fuel efficiency. Even improvements of a few miles per gallon help, said Larrick.

He offered an example: If you trade in a 34 mpg car for one rated at 50 mpg, you reduce gas consumption by about 94 gallons over 10,000 miles.

But if you trade in a car rated at 16 mpg for a model rated at 20 mpg, you reduce gas consumption by 125 gallons over the same distance.

You get “big gains with small changes in big vehicles,” said Larrick.

But the mpg illusion means that consumers scoff at improving mpg ratings at the low end of the efficiency scale — and too readily praise improvements at the higher end of the scale.

Larrick and Soll (who are carpooling friends) conducted studies of hypothetical car purchases based on perceptions of fuel efficiency.

They found that buyers are willing to pay a high cash premium for fuel-efficient cars based on misperceptions of how much fuel is actually saved.

“People are willing to pay too much for a very efficient car,” said Larrick. “But they should also see the value of moving (mpg) out of the teens and into the 20s.

He made his case for gpm to an audience of 25 at the Harvard University Center for the Environment (HUCE). Joining him in a dialogue was behavioral economist Max Bazerman, Jesse Isidor Straus Professor of Business Administration at Harvard Business School.

“There are certain deceptive qualities in mpg,” agreed Bazerman. “Those low miles per gallon numbers all look the same.”

HUCE periodically sponsors such “green conversations” as part of a series of events on the personal and public dimensions of energy usage. OK, admitted Larrick: In a perfect world, everyone would buy a 40 mpg Honda Civic, or a similar high-mileage vehicle.

But the reality is that a lot of American cars already on the road are not as fuel-efficient. So part of the policy over fuel consumption should be making even small improvements in fuel efficiency for less-than-efficient vehicles.

“Small improvements in big cars are good,” said Larrick.

Beyond these small improvements, he advised using buy-back programs and market incentives to phase out the worst gas gulpers — those with mpg ratings in the teens. The idea, cash for clunkers, has already been widely adopted overseas.

European countries, Larrick pointed out, already use a gpm measure of fuel efficiency. (In Great Britain, for instance, a car’s fuel efficiency is expressed as liters per 100 kilometers.)

Adopting a gpm measure underscores the beauty of a cash-for-clunkers plan, he said. Replacing a car that gets 14 mpg with one that gets 25 mpg, for instance, saves 300 gallons of fuel over 10,000 miles — the equivalent of avoiding 3 tons of carbon dioxide going into the atmosphere.

As of January, three cash-for-clunkers bills are on the table in Congress. One would give new car buyers a credit of up to $5,000 for buying a U.S.-made car that gets at least 27 mpg. The car traded in must get worse mileage, must be at least eight years old, and must be junked after the new-car sale.

As for miles per gallon: Keep it, said Larrick. It’s good at least for calculating the number of miles you’ll get out of a car’s gas tank.

“Miles per gallon is a powerful number in the consumer’s imagination,” he said. “It’s simple, meaningful, salient, and ‘fixed’ for a given car.”

There are ways to calculate gpm that are similarly “sticky” for the consumer imagination, said Larrick.

In the meantime, he advised simply adding the gpm figure to standard car listings, like Kelley’s or Consumer Reports magazine — and equipping dealerships with ways to calculate it for the benefit of new buyers.

Adding the unfamiliar gpm mile number is a tough sell, admitted Larrick — analogous to getting Americans to adopt the metric standard. “No one wants to explain that much math to make the switch,” he said.

“We’re not miscalculating” by using just mpg, said Bazerman, who likes the gpm concept. “We’re failing to calculate.”

For more on the gallons-per-mile concept, go to www.mpgillusion.com. And to do a little figuring for your own car, go to www.gpmcalculator.com.

In 1968, the United States was exporting oil. A decade later, given massive increases in domestic demand, it was importing half of this coveted fuel.

By June 1979 this dramatic change — from supplier to buyer — created an oil shock that rolled across the nation.

By the Fourth of July, high prices and low supplies had spawned a national disaster. Members of Congress, facing long gas lines and short tempers, were afraid to go home.

Historian Meg Jacobs, a Radcliffe Fellow this year, is using the lens of this energy crisis to examine governance in a conservative era. In particular, she is looking at how leaders from Ronald Reagan to George W. Bush have reconciled their anti-government ideologies with the demands of actually governing.

Jacobs, who teaches 20th century American history at the Massachusetts Institute of Technology, shared her research in a lecture last week (May 13) at the Radcliffe Gymnasium.

Her forthcoming book, “Panic at the Pump,” uses energy policy as a central metaphor in a history of America’s presumed drift to the right over the past decades.

Jacobs, a one-time postdoctoral fellow at Harvard Business School, tends to look at the past 100 years with an eye on dollars and cents. She is the author of the prize-winning “Pocketbook Politics: Economic Citizenship in Twentieth-Century America” (Princeton University Press, 2005).

After all, Jacobs told her Radcliffe audience of 50, economic issues “are close to the center of changing relationships between citizens and government.” In the past century, she said, Americans have come to expect — to feel entitled to — a solid standard of living, with high wages and stable prices.

When that expectation is shaken, as in the Great Depression, Americans have come to expect — to feel entitled to — dramatic help from the federal government. It is the durability of that expectation, said Jacobs, that still acts as a check on America’s New Right.

Reagan abolished Carter-era checks on oil prices, for instance, she said — but could do little more to dismantle the regulatory machinery of the U.S. Department of Energy (DOE).

The DOE grew out of energy regulations promulgated during a foreshadowing of the oil shock, the Arab-Israeli War of 1973.

When DOE finally blossomed into a full federal agency, it had immediate power and momentum. “On opening day,” said Jacobs, the agency already had 20,000 employees and a budget of $10 billion.

At first, Jacobs thought her book on energy policy and conservative governance would record how the right took apart government. Instead, it became the story of the lasting stability of the federal government’s energy policy.

From Nixon on, she said — in an irony of history — American conservatives “oversaw a massive buildup of government they did not want.”

Jacobs counts among those conservatives President Jimmy Carter, a right-leaning Democrat whose values (and desire for less government) made him a “handmaiden for later Republicans,” she said.

Carter was a former Navy officer who feared the political implications of oil dependence, and whose religious values contained an ethic of conservation.

His response to the oil shock was dramatic and unconventional. On July 15, 1979, he gave a televised address now known as the “malaise speech,” scolding the American public for their lives of excessive consumption and spiritual void. “This is not a message of happiness,” he said, “but a warning.”

It was a failure, said Jacobs. “The public did not want to know they were spoiled and indulgent,” and on the streets the reaction was “panic at the pump.” At the polls, Carter’s ratings sank to 25 percent, lower than Nixon’s in the Watergate era.

Carter’s failure to communicate also muted some of his conservation ideals that today seem prescient. He wanted to raise the price of fossil fuel, encourage energy conservation at home (remember the cardigan sweaters?), and encourage alternative energy sources.

Carter was soon attacked from the left by presidential aspirant Sen. Edward M. Kennedy (D-Mass.), who accused him of being insensitive to high energy prices. And he was attacked from the right by conservatives incensed by the oil crisis — “Exhibit A,” said Jacobs.

Then came the Iran hostage crisis and the Soviet invasion of Afghanistan. But Carter’s real stumble was domestic and economic, she said: a failure to keep oil flowing and prices cheap.

Victorious in 1980, Reagan capitalized on Carter’s failure, said Jacobs, “but that was different than Americans being anti-government.”

Yes: America’s shift to the right is real, she said. It comes from frustrations over issues of property, race, and religion; from a backlash at purported government intrusions (civil rights legislation, Great Society programs, welfare); and from presumed government incompetence (Vietnam, energy shortages).

But the shift to the right has been slowed and complicated by a durable thread in the fabric of American politics not yet fully appreciated, said Jacobs: “the reality of conservative rule in an era of New Deal ideas.”

File Dominick Reuter/Harvard News Office

China has become second only to the United States in its national power-generating capacity and is now the world’s largest CO2 emitter. “The world is struggling with the question of how do you make the switch from carbon-rich fuels to something carbon-free,” said lead author Michael B. McElroy, Gilbert Butler Professor of Environmental Studies. “The real question for the globe is: What alternatives does China have?”

A team of environmental scientists from Harvard and Tsinghua University has demonstrated the enormous potential for wind-generated electricity in China. Using extensive meteorological data and incorporating the Chinese government’s energy-bidding and financial restrictions for delivering wind power, the researchers estimate that wind alone has the potential to meet the country’s electricity demands projected for 2030.

The switch from coal and other fossil fuels to greener wind-based energy could also mitigate CO2 emissions, thereby reducing pollution. The report appeared as a cover story in the Sept. 11 issue of Science.

“The world is struggling with the question of how do you make the switch from carbon-rich fuels to something carbon-free,” said lead author Michael B. McElroy, Gilbert Butler Professor of Environmental Studies at Harvard’s School of Engineering and Applied Sciences (SEAS).

China has become second only to the United States in its national power-generating capacity — 792.5 gigawatts per year with an expected future 10 percent annual increase — and is now the world’s largest CO2 emitter. Thus, added McElroy, “the real question for the globe is: What alternatives does China have?”

While wind-generated energy accounts for only 0.4 percent of China’s total current electricity supply, the country is rapidly becoming the world’s fastest-growing market for wind power, trailing only the United States, Germany, and Spain in terms of installed capacities of existing wind farms.

Development of renewable energy in China, especially wind, received an important boost with passage of the Renewable Energy Law in 2005; the law provides favorable tax status for alternative energy investments. The Chinese government also established a concession bidding process to guarantee a reasonable return for large wind projects.

“To determine the viability of wind-based energy for China we established a location-based economic model, incorporating the bidding process, and calculated the energy cost based on geography,” said co-author Xi Lu, a graduate student in McElroy’s group at SEAS. “Using the same model we also evaluated the total potentials for wind energy that could be realized at a certain cost level.”

Specifically, the researchers used meteorological data from the Goddard Earth Observing Data Assimilation System (GEOS) at NASA. Further, they assumed the wind energy would be produced from a set of land-based 1.5-megawatt turbines operating over non-forested, ice-free, rural areas with a slope of no more than 20 percent.

“By bringing the capabilities of atmospheric science to the study of energy we were able to view the wind resource in a total context,” explained co-author Chris P. Nielsen, executive director of the Harvard China Project, based at SEAS.

The analysis indicated that a network of wind turbines operating at as little as 20 percent of their rated capacity could provide potentially as much as 24.7 petawatt-hours of electricity annually, or more than seven times China’s current consumption. The researchers also determined that wind energy alone, at around 7.6 U.S. cents per kilowatt-hour, could accommodate the country’s entire demand for electricity projected for 2030.

“Wind farms would only need to take up land areas of 0.5 million square kilometers, or regions about three-quarters of the size of Texas. The physical footprints of wind turbines would be even smaller, allowing the areas to remain agricultural,” said Lu.

By contrast, to meet the increased demand for electricity during the next 20 years using fossil fuel-based energy sources, China would have to construct coal-fired power plants that could produce the equivalent of 800 gigawatts of electricity, resulting in a potential increase of 3.5 gigatons of CO2 per year. The use of cleaner wind energy could both meet future demands and, even if only used to supplement existing energy sources, significantly reduce carbon emissions.

Moving to a low-carbon energy future would require China to make an investment of around $900 billion (at current prices) over the same 20-year period. The scientists consider this a large but not unreasonable investment given the present size of the Chinese economy. Moreover, whatever the energy source, the country will need to build and support an expanded energy grid to accommodate the anticipated growth in power demand.

“We are trying to cut into the current defined demand for new electricity generation in China, which is roughly a gigawatt a week — or an enormous 50 gigawatts per year,” said McElroy. “China is bringing on several coal-fire power plants a week. By publicizing the opportunity for a different way to go we will hope to have a positive influence.”

In the coming months, the researchers plan to conduct a more intensive wind study in China, taking advantage of 25-year data with significantly higher spatial resolution for north Asian regions to investigate the geographical year-to-year variations of wind. The model used for assessing China could also be applied for assessing wind potential anywhere in the world, onshore and offshore, and could be extended to solar-generated electricity.

Yuxuan Wang, associate professor in the Department of Environmental Science and Engineering at Tsinghua University in Beijing, also contributed to the study. The team’s research was supported by a grant from the National Science Foundation (NSF).

Figure 1.Climate impacts measured in terms of radiative forcing from the global and U.S. on-road transportation (ORT) and power generation (PG) sectors. The CO₂ radiative forcing shown is for the 20-year time horizon. The sum of total non-CO₂ and CO₂ forcing is indicated above each bar.

Figure 2. Climate impacts measured in terms of radiative forcing of conversion to plug-in hybrid electric vehicle fleet in the U.S. and globally for two replacement energy sources: (S1) zero emissions renewable sources, and (S2) electric power generation sector in current state. The CO₂ radiative forcing shown is for the 20-year time horizon. The sum of total non-CO₂ and CO₂ forcing is indicated above each bar.