Gizmag is always on the lookout for alternative means of powering vehicles and saving precious fossil fuels. But, in truth, the vast majority of us still drive exclusively petrol-powered cars. And the even sadder truth, outlined in a new research from the University of Michigan, is that the average fuel efficiency of a US vehicle has improved only three miles per gallon since the days of the Ford Model T.

Michael Sivak and Omer Tsimhonia, of the University of Michigan Transportation Research Institute, calculated the distance driven and fuel consumed for the entire US fleet of vehicles – incorporating cars, motorcycles, trucks and buses – between 1923 and 2006. Using those numbers, they were then able to analyze fuel efficiency on US roads at any time, and make telling comparisons between eras.

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According to Ford, the Model T – which began mass-production in 1913 – averaged a fairly healthy 25 miles to the gallon. Nonetheless, by 1923, the year the study begins, the average fuel efficiency of the entire US fleet was 14 mpg. That figure remained about the same for more than a decade.

From 1935, however, fuel efficiency fell into steady decline, dropping to an alarming 11.9 mpg in 1973. When you think about the kind of vehicles released through that time – and the number of extras that steadily became standard – it’s almost not surprising. All those fins and chrome and power-assisted systems came with heavy penalties in efficiency. The Environmental Defense Fund, for example, estimates that air-conditioning alone decreases the fuel efficiency of a car by as much as 12%.

But, with the fuel crisis of the 70’s, fleet efficiency was compelled to improve, and in a hurry. From 1974 the economy of the US fleet improved five miles a gallon to 16.9 mpg in 1991. Curiously, since then – despite growing environmental awareness and publicly-voiced concern – improvement has been painfully slow, reaching just 17.2 mpg in 2006.

The underlying problem in recent years, of course, isn’t with new cars, some of which can achieve close to 40 mpg. It’s all those old vehicles out there, chewing up gas like there’s no tomorrow. As far as Sivak and Tsimhoni are concerned, it’s much more important to improve fuel efficiency at this end – from 15 to 16 mpg, say – than trying to get a Prius from 40 to 41 mpg.

By their estimates, for the US to reduce its total annual fuel consumption by 10%, fuel efficiency across the entire fleet of cars, motorbikes, truck and buses would have to rise nearly two percent. That may not sound like much, but it took 15 years for a 0.3% change. And when you’re talking about 10,000 pounds of carbon dioxide from the average medium-sized sedan, making a change has never seemed more critical.