More fuel efficient vehicles, like Ford's C-Max line-up, have helped reduce the amount of gasoline used by Americans in the last three decades.

A new study from the University of Michigan examines one of the important factors that have undermined the price of oil and gasoline: Americans are using less gas.

The price of gasoline and crude oil has plunged since last summer and the role of new discoveries in North American shale formations has been highlighted.

Over the past two years, oil production in the U.S. has reached levels last seen in the late 1960s and early 1970s. Average fuel consumption by American drivers is at its lowest level in at least 30 years, according to a University of Michigan researcher.

But the supply side is only one half of the equation, the demand side has also dropped as a combination of higher fuel prices, stiffer fuel economy standards, which have driven major changes in automotive engineering and shifts in consumer preferences have also played a role.

In a follow-up to a series of reports released over the past two years, Michael Sivak of the U-M Transportation Research Institute (UMTRI) examined recent trends in fuel consumption by cars, pickup trucks, SUVs and vans in the U.S. from 1984 to 2013.

His findings show that gallons of gasoline consumed per person, driver, vehicle and household in the U.S. have dropped below 1984 levels — the first year of the study — and down 14-19% from peak levels a little more than a decade ago in 2003 and 2004.

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“Despite population growth of 8%, the absolute amount of fuel consumed by light-duty vehicles decreased by 11% during the period 2004 — the year of maximum consumption — through 2013,” said Sivak, a research professor at UMTRI and director of the Sustainable Worldwide Transportation research consortium.

Sivak found that in 2013 the amount of fuel consumed was about 392 gallons per person (down 17% from 2004), 583 gallons per driver (down 16% from 2004), 524 gallons per vehicle (down 14% from 2003) and 1,011 gallons per household (down 19% from 2004).

In 1984, annual fuel-consumption rates were 400 gallons per person, 608 gallons per driver, 602 gallons per vehicle and 1,106 gallons per household.

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In addition to examining changes in fuel consumption, the study also analyzed changes in the number of vehicles and distance driven. For both measures, rates per person, driver, vehicle and household are at their lowest levels since the 1990s.

Sivak said that although economic factors likely contributed to declining rates of vehicle ownership and distance driven, other societal changes such as increased telecommuting, use of public transportation and urbanization of the population, along with changes in the age composition of drivers, have influenced the need for personal transportation.

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“The reductions in the fuel-consumption rates reflect, in part, the added contribution of the improvements in vehicle fuel economy,” Sivak said. “Overall, the combined evidence from this and the previous studies indicates that—per person, per driver and per household—we now have fewer light-duty vehicles, we drive each of them less and we consume less fuel than in the past. There is no evidence in the 2013 data that the peaks in the rates that we experienced about 10 years ago were temporary.”