That's the implication of a new study that says Americans are burning nearly 1 billion more gallons of gasoline each year than they did in 1960 because of their expanding waistlines. Simply put, more weight in the car means lower gas mileage.
Using recent gas prices of $2.20 a gallon, that translates to about $2.2 billion more spent on gas each year.
"The bottom line is that our hunger for food and our hunger for oil are not independent. There is a relationship between the two," said University of Illinois researcher Sheldon Jacobson, a study co-author.
"If a person reduces the weight in their car, either by removing excess baggage, carrying around less weight in their trunk, or yes, even losing weight, they will indeed see a drop in their fuel consumption."
The lost mileage is pretty small for any single driver. Jacobson said the typical driver -- someone who records less than 12,000 miles annually-- would use roughly 18 fewer gallons of gas over the course of a year by losing 100 pounds. At $2.20 per gallon, that would be a savings of almost $40.
Outside experts said that even if the calculations aren't exact, the study makes sense.
"If you put more weight into your car, you're going to get fewer miles per gallon," Emory University health care analyst Kenneth Thorpe said Wednesday.
The same effect has been seen in airplanes. Research from the Centers for Disease Control and Prevention found that heavy fliers have contributed to higher fuel costs for airlines.
The obesity rate among U.S. adults doubled from 1987 to 2003, from about 15 percent to more than 30 percent. Also, the average weight for American men was 191 pounds in 2002 and 164 pounds for women, about 25 pounds heavier than in 1960, government figures show.
The study's conclusions are based on those weight figures and Americans' 2003 driving habits, involving roughly 223 million cars and light trucks nationwide.
It will appear in the October-December issue of The Engineering Economist, a peer-reviewed journal published by the American Society of Engineering Education and the Institute of Industrial Engineers.
Jacobson, an industrial engineer, conducted the research with Laura McLay, a doctoral student in his Champaign-Urbana lab who now works at Virginia Commonwealth University.
They estimated that more than 39 million gallons of fuel are used each year for every additional pound of passenger weight.
The amount of extra fuel consumption blamed on weight gain since 1960 -- 938 million gallons -- would fill almost 2 million cars with gas for an entire year. However, that is only 0.7 percent of the total amount of fuel consumed by U.S. passenger vehicles each year, Jacobson said.
The estimates "are probably pretty reliable," said Larry Chavis, an economist at the University of North Carolina in Chapel Hill. "I don't know if it's going to encourage anybody to go out and lose weight to save gasoline, but even for individual families, it could have an effect on their budget."
Dr. Jeffrey Koplan, former CDC director and chairman of an Institute of Medicine report on obesity, said the findings are almost beside the point.
"The wrong fuel is being focused on," said Koplan, now at Emory University. "If you're heavier, the most important fuel you use more of is food."
Eating less, driving less and choosing more active means of transportation would reduce gas consumption, and also help reverse rising obesity rates, he said.
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