American new car buyers are buying more fuel efficient vehicles on average, but declining gasoline prices may be drawing purchasers back to larger vehicles and trucks once again.

According to researchers at the University of Michigan Transportation Research Institute (UMTRI) who regularly track average fuel economy of new vehicle purchases in the U.S., gas mileage of new vehicles sold in the U.S. remained unchanged in October.

UMTRI data showed that the combined average fuel economy of cars, light trucks, vans and SUVs purchased in October was 25.3 mpg. The average was the same as it was in September, but down from 25.8 mpg in August.

Since October, 2007 when average fuel economy of new vehicle purchases was first monitored by UMTRI, vehicle fuel economy is up by 5.2 mpg.

See Also: Top 10 Most Fuel Efficient Non-Hybrid Cars – 2014

According to Dr. Michael Sivak of UMTRI, the unchanged average fuel economy is likely a net consequence of two opposing trends—less demand for more fuel-efficient vehicles because of the decreasing price of gasoline, and improved fuel economy of 2015 model year vehicles compared to 2014 model year vehicles.

U.S. sales of new cars, light trucks, vans and SUVs in October totalled just under 1.7 million units. The top three selling vehicles from October were the Ford F-Series, the Chevrolet Silverado and the Dodge Ram.

According to the U.S. Energy and Information Administration, weekly U.S. regular conventional retail gasoline prices have gone from a high of $3.64 per gallon in late June, down to $3.02 at the end of October.

In addition to average fuel economy, UMTRI also issued a monthly update of their national Eco-Driving Index, which estimates the average monthly emissions generated by an individual U.S. driver. The EDI takes into account both the fuel used per distance driven and the amount of driving—the latter relying on data that are published with a two-month lag.

The EDI improved for the fourth consecutive month in August to a record-low 0.76 (the lower the value, the better). The index currently shows that emissions of greenhouse gases per driver of newly purchased vehicles have dropped 24 percent overall, since October 2007 when tracking first began.