The average fuel economy in new vehicle sales increased to its highest point in the past nine months in May, according to researchers at the University of Michigan Transportation Research Institute.

Fuel economy for new cars, light trucks, SUVs, and vans was up to 25.5 miles per gallon, an increase from the average of 25.2 miles per gallon in April. This figure indicates the window sticker values displayed on new vehicles rather than the unadjusted Corporate Average Fuel Economy values.

May's figure was the highest average fuel economy in new vehicle sales since August. The average vehicle sold in that month had a fuel economy rating of 25.8 miles per gallon.

The figure fell to 25.3 miles per gallon for the next three months after August and again to 25 miles per gallon in December. In 2015, the average fuel economy in new vehicle sales has increased in three months and decreased in two.

"The improvement likely reflects the increased price of gasoline in May," said Michael Sivak, a research professor at UMTRI.

Sivak and colleague Brandon Schoettle have been monitoring fuel economy since October of 2007. The average vehicle sold in that month got 20.1 miles per gallon.

The average figure is derived from the sales figures of different vehicle models as well as their ratings in the Environmental Protection Agency's "Fuel Economy Guide." The researchers estimate that fuel economy information is available for 99.7 percent of the vehicles sold each month.

The researchers also average the fuel economy of sales between October and September to determine an average fuel economy by model year. Although model year averages improved steadily between 2008 and 2014, the average for the 2015 model year is currently the same as the 2014 model year at 25.3 miles per gallon.

In addition to the fuel economy measurements, Sivak and Schoettle update their "Eco-Driving Index" each month. This figure estimates the average monthly emissions generated by the average individual driver in the United States. This figure is issued on a two-month lag, the same delay the Federal Highway Administration has in issuing monthly travel data.

In March, the Eco-Driving Index stayed at 0.82 for the third consecutive month. This number indicates that the emissions levels produced per driver of newly purchased vehicles were 18 percent lower than the baseline levels in October of 2007.

The Eco-Driving Index uses figures on both the miles driven and the fuel used per distance to come to this number. Although the number of miles traveled in March was 3 percent higher than the total in October of 2007, the fuel used per miles driven was 20 percent lower than in October of 2007.