Merely 3 miles per gallon improvement in 80 years.
The Transportation Research Institute calculated mileage for the entire US fleet of vehicles from 1923 to 2006.
The 1913 Model T averaged 25 miles to the gallon; the entire US fleet was 14 mpg. From 1935 to 1973 fuel efficiency fell to 12 mpg, rose to 16.9 mpg in 1991, and reached 17.2 mpg by 2006.