The average fuel economy in new vehicles purchased in October was slightly worse than the previous month, according to the University of Michigan Transportation Research Institute. However, the figure also marked the fourth consecutive month of year-over-year improvement.

Vehicles sold in October had an average combined city and highway fuel economy of 25.2 miles per gallon. This was down from 25.3 miles per gallon in both August and September, but up from 25 miles per gallon in October 2016.

UMTRI researchers Michael Sivak and Brandon Schoettle have been tracking changes in sales-weighted fuel economy since October 2007, when the average new vehicle got 20.1 miles per gallon. The figures are derived from monthly sales figures as well as information on the efficiency of cars, pickup trucks, SUVs, and vans published in the Environmental Protection Agency's "Fuel Economy Guide."

Average sales-weighted fuel economy peaked at 25.5 miles per gallon in August 2014. It has remained fairly steady, varying between 24.8 miles per gallon and 25.4 miles per gallon, since that point. However, the 2017 model year average—measuring figures between October and September—stood at 25.2 miles per gallon, up from 25.1 miles per gallon in the previous year model years.

UMTRI also updated its Eco-Driving Index, a figure estimating the average greenhouse gas emissions produced by a driver of a new light-duty vehicles in the United States. This figure estimates how the emissions compare to the October 2007 baseline levels, and is issued on a two-month delay due to a corresponding lag in information from the Federal Highway Administration.

The Eco-Driving Index for August was 0.82, indicating an 18 percent reduction in greenhouse gas emissions from the baseline levels. This was up from 0.81 in July and 0.8 in August 2016. The lowest index recorded was 0.78, in November 2013.

For the second month in a row, vehicle miles traveled were 1 percent higher than in October 2007. The average driver in August used 19 percent less fuel per distance driven than baseline levels.