In 2008, half of new-car buyers in the U.S. bought vehicles that were rated at less than 20 mpg. Today, just over a quarter do so.

Building on research that shows average fuel economy has improved 4.5 mpg between model years 2008 and 2014, University of Michigan researchers Michael Sivak and Brandon Schoettle document sales-weighted distributions for the same model year vehicles (cars, pickup trucks, vans and SUVs).

Their new report found that improvements are present throughout the distributions of vehicle fuel economy.

About 24 percent of consumers bought new 2008 vehicles with fuel economy between 11 mpg and 17 mpg, and 26 percent purchased vehicles with mpg between 17 and 20. Six years later, less than 9 percent of car buyers bought a new 2014 model with fuel economy less than 17 mpg. Another 19 percent drove new cars that averaged between 17 mpg and 20 mpg.

While about 35 percent of new vehicles sold in 2008 had average fuel economy between 20 mpg and 26 mpg, compared to roughly 31 percent for model year 2014, huge improvements were made in the sales of fuel-efficient cars, the researchers say.

Nearly 41 percent of new-car buyers bought 2014 vehicles with mpg of at least 26, including 27 percent who purchased vehicles averaging at least 30 mpg. In 2008, the corresponding figures were 15 percent and 5 percent, respectively.

Overall, average fuel economy for light-duty vehicles improved from 20.8 mpg for model year 2008 to 25.3 mpg for model year 2014.