Researchers looked at the miles per gallon for all vehicles on the road for every year since 1923. The data included cars and all manner of trucks. Since actual mpg data wasn't available for every year they used distances driven and fuel consumed to calculate the actual, on-road fuel economy.
They found that the average fuel economy of the fleet of vehicles in 1923 was about 14 miles per gallon. Then fuel economy actually decreased over the decades to a paltry 11.9 in 1973. Between the '20s and '70s, with very inexpensive gasoline, Americans were somewhat indifferent to the fuel economy of their cars. It took the '73 gas crisis to help convince the country to start driving smarter, more efficient vehicles. Indeed, the numbers begin to climb in 1974, eventually hitting 16.9 average mpg in 1991. Since then improvements seem to have stalled. The overall fuel efficiency for vehicles on the road in 2013 was only 17.6 mpg.
Like all shocking findings from scientific studies, this one is a little tricky. The way fuel consumption has been calculated over the years has change several times. From 1923 to 1935, for instance, fuel-economy information is available only for the entire fleet of all vehicles. That includes cars, trucks, buses and motorcycles lumped into one category. From 1936 to 1965 there were separate estimates for cars and trucks, but motorcycles were lumped in with cars and it wasn't until 1966 that the Department of Transportation began to distinguish different weights of truck, from lightweight to heavy duty.
So while on-road fuel economy for cars decreased from 1936 to 1973 from 15.3 mpg to 13.4, cars actually boasted 23.4 mpg in 2013, a 10 mpg increase over the last four decades. Lightweight vehicles in general have gained nearly 8.7 mpg since 1966. Medium and heavy-duty trucks saw the least improvement, from 5.6 mpg in 1966 to 6.4 mpg in 2013. The authors of the study note, however, that it should be taken into account that medium and heavy duty trucks can now haul a lot more stuff than in previous decades.