Following a marked improvement in July, the average fuel economy among vehicles sold in the United States dropped back slightly in August.

According to researchers at the University of Michigan Transportation Research Institute, the typical combined fuel economy for a vehicle sold during this month was 25.3 miles per gallon. This was down from 25.4 miles per gallon in the previous month, but up from 25.1 miles per gallon in August 2016.

UMTRI researchers Michael Sivak and Brandon Schoettle have been tracking fuel economy trends in U.S. vehicle purchases since October 2007, when the average vehicle got 20.1 miles per gallon. Sivak and Schoettle use monthly sales data as well as the window sticker values for combined city and highway mileage from the Environmental Protection Agency's "Fuel Economy Guide" to determine the typical fuel economy of cars, light trucks, SUVs, and vans sold each month.

The average sales-weighted fuel economy increased gradually from the start of the research, reaching a high of 25.5 miles per gallon in August 2014. The model year average, which takes into account fuel economy figures from October to September, climbed from 20.8 miles per gallon for the 2008 model year average to 25.1 miles per gallon for the 2014 model year average.

Improvements to sales-weighted fuel economy stalled in late 2014, when gasoline prices began to drop. The model year average remained unchanged in 2015 and 2016, and the average fuel economy in newly purchased vehicles wavered between 24.8 miles per gallon and 25.3 miles per gallon until the improvement in July. Sivak said the high average for that month was a result of light trucks making up a smaller share of the month's sales.

In addition to the fuel economy research, Sivak and Schoettle track changes to a figure called the Eco-Driving Index. This compares the estimated greenhouse gas emissions generated by new vehicle drivers to those of drivers in October 2007. The update is issued on a two-month delay due to a corresponding lag in data from the Federal Highway Administration.

The Eco-Driving Index for June stood at 0.83, indicating that emissions were down 17 percent from baseline levels. This was up from 0.82 in May and unchanged from June 2016. The lowest Eco-Driving Index, 0.78, was recorded in November 2013.

In June, the number of vehicle miles traveled in the U.S. was 3 percent higher than in October 2007. The estimated fuel used per distance driven was 19 percent lower than in that month.