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.
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.
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.
"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.
President Obama announced in May that new cars should average 35.5 mpg (15.09 km/l) by 2016.