What’s the difference between fuel efficiency and fuel economy?

What we really ought to be thinking about is fuel consumption, measured in gallons per mile — not the customary miles per gallon…

By Lori Baker

Fuel economy is a well-defined measure familiar to anybody who has bought a car in the U.S.: it means miles per gallon. Fuel efficiency, on the other hand, is a looser, descriptive term referring to how efficiently a vehicle uses fuel, according to John Heywood, Professor of Mechanical Engineering and past Director of the Sloan Automotive Laboratory.

Heywood, whose research looks at potential strategies for lowering transportation’s fuel consumption and greenhouse gas emissions, notes that both terms are important in thinking about how to change the U.S.’s gas-hungry car culture. What we really ought to be thinking about, he says, is fuel consumption, measured in gallons per mile — not the customary miles per gallon. This measure “tells me exactly how many gallons of gas I use if I drive 100 miles,” Heywood says. “If we are trying to reduce fuel consumption, this is what we need to look at.”

Thinking about miles per gallon, Heywood says, can actually confuse matters and make small improvements seem bigger than they really are. “If I am driving to New York City and back in a car that gets 40 mpg, and I improve the mileage so the car gets 60,” he says, “that is a difference — it’s an improvement and it sounds impressive, but really, either way, I’m not using all that much fuel.” On the other hand, he says, if you start with an SUV that uses 10 mpg, and improve it to 15 mpg, the difference in the overall amount of fuel consumed is much greater, a reduction of seventeen gallons over the 500-mile trip, versus just over four gallons. One way to begin to get people thinking about fuel consumption might be to urge auto manufacturers to post a “gallons-per-100-miles” measure alongside the more familiar mpg rating. “That might start getting people more connected with thinking about how much fuel they are actually using,” Heywood says.

Though consumers are becoming more attuned to their fuel consumption, Heywood says, they are less accustomed to thinking about it in percentages, interpreting numbers with decimal points, and understanding inverse mileage proportions — and change isn’t likely to come easily. In the U.S. and other developed nations, people are aware of the need to reduce fuel consumption, but they also like the transportation systems they have — and the plentiful, cheap fuel that continues to make possible the ever- bigger, heavier, higher-performance cars they drive. The great irony, Heywood points out, is that even though the large engines powering heavy vehicles are as efficient as the engines of smaller cars, the weight and drag of the vehicles they’re hauling around dramatically worsens their overall fuel economy.

Reducing our fuel consumption, developing hybrid and electric cars, and establishing access to alternative fuels, will be of vital environmental and economic importance over the next ten to fifteen years, Heywood says. Developing nations like China and India are poised to create a huge demand for fuel, which is likely to have a dramatic effect on the price of gas for everyone else. “I was in Los Angeles recently, and saw the highways there packed with big SUVs,” he says, “and I thought, what is L.A. going to do when gas costs $10 to $15 a gallon?”

Posted: October 5, 2010

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