I hope he's giving a context to his quanta.
The car's were capable of over 27-mpg in 1991,but weather,traffic jams,'n such drove the actual mpg down.
Highways have changed over time.
Speed limits have changed.
Urban population density has changed.
Profligate increases in un-synchronized traffic controls.
Incidence of highway-closing wrecks.
Actual driving speeds versus posted speeds.
Mechanical efficiencies of engines/powertrains/tires
Aerodynamics
Oxygenated fuels
Flex fuels
Gasoline vs Diesel
etc.
Some technologies didn't even exist in 1923
