Fuel economy has seen a gradual increase in light-duty vehicles since the 1990s with a 1.8 mpg improvement in 2013, according to the University of Michigan's Transportation Research Institute (UMTRI). 

With the introduction of fuel saving technologies in the late 90s, light-duty vehicles have improved on-road fuel economy slower than expected. All light-duty vehicles saw a 10.2-percent improvement in fuel economy from 1991 to 2013, according to the study. 

The fuel-economy data for cars shows a similar pattern when compared to all vehicles. Fuel economy decreased by 1.9 mpg from 1936 to 1973 and saw a recovery period from 1973 to 1991 (13.4 mpg to 21.2 mpg). For the next 22 years, car fuel economy increased gradually. 

The report produced by the institute's Michael Sivak and Brandon Schoettle analyzes the changes in fuel economy of vehicles from 1923 to 2015. 

According to the study, a key issue with improving fuel economy lies in turnonver of vehicles on the road. Improvements in fuel economy of new vehicles takes time to make an impact on the overall fuel economy. 

The observations suggest that the focus should be on the lower tails of the distributions of fuel economy in each vehicle class, according to the report. 

Information about distances driven and fuel consumed was used to calculate the actual, on-road fuel economy for all vehicles and for different classes of vehicles, with primary interest in light-duty vehicles (cars, pickup trucks, vans, and SUVs).

The findings this study presents serve as an update to a research report conducted in 2009. The previous study looked at actual, on-road vehicle fuel economy in the U.S. from 1923 to 2006. Sivak and Schoettle's study adds information such as retroactive adjustments that were made to different values by the U.S. Department of Transportation. 

View the study's abstract for more information.