>have been accomplished almost completely by reducing the weight of the cars This is entirely false. All the work in materials to make cars lighter has been co

>have been accomplished almost completely by reducing the weight of the cars This is entirely false. All the work in materials to make cars lighter has been countered by added amenities (from power seats all the way to chilled cupholders), additional safety measures (extra airbags), and (in the U.S. at least) shifting preferences toward trucks and SUVs. From the first google result on the subject, slate: "The average new car weighed 3,221 pounds in 1987 but 4,009 pounds in 2010." Much of efficiency gains are coming from improved engines which have better control of the combustion process to make sure fuel burns more completely and less energy is wasted, or in other words, engines are better quality.