Americans are pumping more gas per year because of increased body weight.
Experts say that carrying around an extra 100 pounds in the trunk can cut a vehicle's gas mileage by about 2 percent.
But what about that extra weight carried up in the passenger compartment?
A new study calculated that Americans are pumping nearly 1 billion gallons of gasoline a year more than they were in 1960 strictly as a result of increased human body weight.
Sheldon Jacobson, director of the simulation and optimization laboratory at the University of Illinois-Champaign, watched gasoline prices break through the 3-a-gallon range last fall and wondered what effect human inflation might be having on demand.
"We felt that beyond public health, being overweight has many other socioeconomic implications," said Jacobson. He challenged Laura McLay, then a doctoral student in his lab and now on the faculty at Virginia Commonwealth University, to try to develop a formula that could measure the impact of extra pounds on fuel use.
Weight on the rise
Writing in the October-December issue of The Engineering Economist, the scientists concluded that each extra pound of body weight carried in all of today's passenger vehicles results in the need for more than 39 million additional gallons of gasoline a year.
And with the weight of the average American having increased by 24 pounds between 1960 and 2002, according to federal data, that translates into 938 million gallons more fuel consumed each year than in 1960.
That's nearly three times the total amount of fuel consumed by all passenger vehicles in the United States each day under current driving habits, the researchers noted.
Based on height-to-weight ratios, the government estimates that nearly two-thirds of American adults were overweight, and almost a third were considered obese.
At 3 a gallon, the tab for extra weight carried on people in cars comes to 7.7 million a day, or 2.8 billion a year, the study found.
Jacobson and McLay said they only used fuel-consumption calculations based on driving data from 2003 for "non-commercial" driving with cars and light trucks, and didn't try to factor in other factors in fuel economy loss, such as increased cargo weights or poor vehicle maintenance, or even that the bigger people are driving bigger cars.
"If anything, our numbers are conservative, because they don't consider indirect obesity costs or increases in miles being driven by more people," Jacobson said.
"Although the amount of fuel consumed as a result of the rising prevalence of obesity is small compared to the increase in the amount of fuel consumed stemming from other factors, such as increased car reliance and an increase in the number of drivers ... it still represents a large amount of fuel and will become even more significant as the rate of obesity increases," the researchers wrote.