Average window-sticker fuel economy of new vehicles sold in the U.S. dropped 0.2 mpg last month from May, according to a monthly study.

Decreasing gasoline prices and the popularity of SUVs, crossovers, and pickups were behind the steep drop in fuel efficiency of vehicles sold. Data comes from the monthly report written by Brandon Schoettle and Michael Sivak of Transportation Research Institute, University of Michigan.

The study reports that sales-weighted average fuel economy hit 25.1 mpg in June. That number was up 5.0 mpg since the UMTRI data was first collected in October 2007. It’s down 0.4 mpg since UMTRI reported a peak of 25.5 mpg in August 2014.

June saw gas prices decline, which helped tip the scale further away from cars and toward trucks.

Light-duty trucks – consisting of pickups, SUVs, crossovers, and vans – made for 933,451 units sold in the U.S. during June. At 540,590 units sold, cars made for only 36.6 percent of U.S. new vehicle sales last month. Car sales were down 13.2 percent from June 2016 while light-duty trucks were up 4.2 percent from a year ago, according to Autodata.

The most recent analysis of greenhouse gas emissions in the university study tracked April 2017 data. Called the University of Michigan Eco-Driving Index (EDI), the index found that average monthly emissions of GHG improved to 0.84 in April, down from 0.85 in March.

The EDI part of the study found that new-vehicle drivers produced 16 percent lower emissions in April 2007 since October 2007. That was 6 percent higher than the study’s record low found in November 2013.

The research study determines EDI based on vehicle fuel economy and mileage driven. Mileage data comes from data published with a two-month lag, which is how the April data was reported with the June findings.

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Consumer Federation of America expects mileage to improve even though consumers are buying more light-duty pickups. Based on a CFA survey released last month, consumers have been choosing more fuel efficient crossovers, SUVs, and pickups over less efficient light trucks in their purchases in recent years.

That’s not enough to tip the scales back to higher fuel efficiency in the UMTRI study.

Automakers expect to see battery electric vehicles, and their high Environmental Protection Agency mileage ratings, bringing up fuel economy and dropping emissions. But not anytime soon in the U.S.

Sales of plug-in vehicles, including all-electric and plug-in hybrid, continue to hover around the 1 percent mark in the U.S.

Automakers such as Tesla, General Motors, Volkswagen, BMW, and Daimler, are preparing for the global sales volume of BEVs to increase quite a lot over the next decade. Sales growth may take place beyond the U.S. if the Trump administration reduces the second phase of the federal fuel economy mandates next year, which is expected to be the case.

It’s also tied into the Trump administration backing out of the Paris climate accord, which was anticipated long before last month’s announcement.

China and Europe may see more of the BEV production volume and sales than the U.S. in years to come.

Long-range BEVs, competitive pricing, and a diverse selection of new electric vehicles by brand and vehicle class should also make a difference in U.S. sales, and in other global markets.