No change in fuel efficiency despite modern technological advances (Image: © 2008 Ford Motor Company)

The average fuel efficiency of the US vehicle fleet has risen by just 3 miles per gallon since the days of the Ford Model T, and has barely shifted at all since 1991.

Those are the conclusions reached by Michael Sivak and Omer Tsimhoni at the University of Michigan Transportation Research Institute in Ann Arbor. They analysed the fuel efficiency of the entire US vehicle fleet of cars, motorcycles, trucks and buses from 1923 to 2006.

They found that from 1923 to 1935 fuel efficiency hovered around 14 mpg (5.95 km/l), but then fell gradually to a nadir of only 11.9 mpg (5.08 km/l) in 1973. By 1991, however, the efficiency of the total fleet had risen by 42 per cent on 1973 levels to 16.9 mpg (7.18 km/l), a compound annual rate of 2 per cent.


The improvements made up to 1991 were in response to two international events – the 1973 oil embargo by the Organisation of Petroleum Exporting Countries (OPEC), and the Iranian Revolution of 1979.

‘No external prods’

Both events disrupted oil supplies to the west, says John DeCicco, former senior auto issues expert at the campaign group Environmental Defense Fund, who was not involved in the new study.

Progress has stalled since then, though, despite growing environmental concerns. From 1991 to 2006 the average efficiency improved by only 1.8 per cent to 17.2 mpg (7.31 km/l). “We were in a period of complacency [during the 1990s]. There were no external prods to improve fuel economy,” says DeCicco.

“By the mid-90s there was only us and the Sierra Club [a US grassroots environmental organisation] pressing for better standards,” DeCicco says. If manufacturers addressed energy concerns at all, it was in the pursuit of “technological fantasies” like the electric car, he says.

Electric vehicle research continues to advance and now has high-profile governmental backing, but it’s unlikely to have a short-term impact on average fuel efficiency in the US.

If the country’s transportation fuel consumption is to fall by 10 per cent, Sivak and Tsimhoni calculate that the average fuel efficiency across the entire fleet will have to rise to 19.1 mpg (8.12 km/l).

Financial incentives

President Obama announced in May that new cars should average 35.5 mpg (15.09 km/l) by 2016. But improvements to new cars could still leave the efficiency of the entire vehicle fleet largely unaffected without changes in policy, say Sivak and Tsimhoni.

They think there should be a financial incentive to promote owners of older vehicles to scrap them in favour of new cars, as happens already in some European countries. In addition, tax breaks should be given to encourage the development and introduction of fuel-saving technologies – particularly in the most fuel-hungry vehicles in each class.

“Society has much more to gain from improving a car from 15 to 16 mpg (6.38 to 6.8 km/l) than from improving a car from 40 to 41 mpg (17 to 17.43 km/l),” they write in their paper. “Similarly, the benefits are greater from improving a truck from 4 to 4.5 mpg (1.7 to 1.92 km/l) than from improving a truck from 7 to 7.5 mpg (3.19 km/l).”

Journal reference: Energy Policy (DOI: 10.1016/j.enpol.2009.04.001)