The average fuel economy in vehicles sold in July was up significantly, reaching its highest point in nearly three years.

According to the University of Michigan Transportation Research Institute, the sales-weighted combined fuel economy of vehicles sold in the United States during the month was 25.4 miles per gallon. This was up from 25.1 miles per gallon in June and 25.2 miles per gallon in July 2016.

This was the second highest average fuel economy recorded since UMTRI began tracking this data in October 2007. The highest figure recorded was 25.5 miles per gallon in August 2014.

"This increase likely reflects the decreased proportion of light trucks in the sales mix in July compared to June," said UMTRI researcher Michael Sivak.

Since August 2014, the average fuel economy of newly purchased vehicles has varied between 24.8 miles per gallon and 25.3 miles per gallon. The fuel economy average had risen gradually since October 2007, when the typical new vehicle got 20.1 miles per gallon, but held fairly steady after gas prices plummeted in late 2014. The model year average, or annual fuel economy average for October through September, improved each year from 2008 to 2014 but stayed at 25.1 miles per gallon from 2014 to 2016.

Sivak and fellow researcher Brandon Schoettle determine the sales-weighted fuel economy by analyzing monthly sales figures as well as fuel economy figures published in the Environmental Protection Agency's "Fuel Economy Guide." Fuel economy information is available for approximately 99.7 percent of all cars, light trucks, SUVs, and vans purchased in the United States.

Sivak and Schoettle also issue a monthly update to their Eco-Driving Index, which estimates the level of greenhouse gas emissions generated by drivers of newly purchased vehicles. The index is updated on a two-month delay due to a corresponding lag in information from the Federal Highway Administration.

The Eco-Driving Index for May stood at 0.82, indicating that drivers of new vehicles were generating 18 percent fewer emissions than comparable drivers in October 2007. This was an improvement from a figure of 0.84 in April, but up from a figure of 0.81 in May 2016. The best Eco-Driving Index recorded in the index was 0.78 in November 2013, indicating that the average driver of a new vehicle was generating 22 percent fewer emissions than a comparable driver in October 2007.

The index is based on travel data and the average amount of fuel used per distance driven. Drivers in May traveled 3 percent farther than those in October 2007 and used 20 percent less fuel per distance driven.