The average fuel economy in new vehicle sales dropped two percent a month after reaching its highest point in seven years, according to researchers at the University of Michigan Transportation Research Institute.

The average fuel economy for light duty vehicles purchased in September-including cars, light trucks, vans, and SUVs-was 25.3 miles per gallon. This was a drop from 25.8 miles per gallon in August, the highest recorded average fuel economy since UMTRI researchers Brandon Schoettle and Michael Sivak began keeping track in October of 2007. The institute said the average fuel economy hasn't dropped by half a mile per gallon in a month since December of 2011.

"This large drop likely reflects the increased sales of light trucks and SUVs, and the reduced demand for fuel efficient vehicles of all types because of falling gas prices," said Sivak.

However, the institute notes that the average fuel economy in new vehicle sales has been above 25 miles per gallon for eight months in a row. In January, researchers put the average fuel economy at 24.9 miles per gallon.

When researchers began keeping track of this data in October of 2007, the average fuel economy in new vehicle sales was 20.1 miles per gallon. The average fuel economy of 2008 model vehicles-including vehicles purchased between October and December of 2007-was 20.8 miles per gallon. The average has increased in each model year, and the 2014 model year average currently stands at 25.3 miles per gallon.

The study calculates fuel economy based on the combined city and highway fuel economy ratings published in the Environmental Protection Agency's Fuel Economy Guide as well as the monthly sales of individual models of light duty vehicles. The study has been averaging the ratings in instances where multiple fuel economy figures are given for a vehicle model, using the most recently available model year data if a vehicle's fuel economy is not listed in the EPA guide, and aggregating ratings and sales figures for vehicles with low sales volume such as luxury cars. Researchers said fuel economy information was available for 99.8 percent of all vehicles purchased.

In addition to the fuel economy ratings, UMTRI has been tracking an "Eco-Driving Index" to measure changes in distance driven and fuel used per distance driven. The index uses this information-some of which is available after a two-month lag-to compare the average monthly vehicle emissions generated by an individual American driver with the levels at the start of research in October of 2007.

The Eco-Driving Index hit a record low for July, standing at 0.77 for the month. This represents a 23 percent decline from the baseline emissions in October of 2007. During the month, distance traveled was at 95 percent of October of 2007 levels and the use of fuel per distance driven was at 81 percent of those levels.

The Eco-Driving Index has fallen for three months in a row; emissions have not risen above October 2007 levels since the start of data collection, and the distance traveled has remained steady between 93 percent and 101 percent of the baseline levels. The amount of fuel used per distance driven has remained at 81 percent of October 2007 levels since February.