Fuel efficiency of vehicles on US roads: 1923-2006
 

Sivak M., Tsimhoni O. (2009).
Fuel efficiency of vehicles on US roads: 1923-2006.
Energy Policy. 37:8:pp.3168-3170.
Web: Fuel efficiency of vehicles on US roads: 1923?2006

Unfortunately it is behind a paywall. Here is the abstract:

This article documents and analyzes the changes in fuel efficiency of vehicles on US roads between 1923 and 2006. Information about distances driven and fuel consumed was used to calculate the on-the-road fuel efficiency of the overall fleet and of different classes of vehicles. The overall fleet fuel efficiency decreased from 14 mpg in 1923 to 11.9 mpg in 1973. Starting in 1974, efficiency increased rapidly to 16.9 mpg in 1991. Thereafter, improvements have been small, with efficiency reaching 17.2 mpg in 2006. The information for 2006 was used to calculate the fuel-efficiency improvements in different classes of vehicles that would be needed to achieve a given percentage reduction in the total amount of fuel consumed by all vehicles.

article
 

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:p

People seem to 'forget' there was a L-O-N-G time when highway speeds were federally limited to 55 MPH (and still legally are, but states aren't enforcing).