Lagging sales of hybrids and diesels don't help either.

New-vehicle fuel economy in the US edged down slightly in June as the market share of SUVs and pickup trucks rose while hybrids and plug-in vehicle demand likely fell, according researchers at the University of Michigan Transportation Research Institute (UMTRI). New light-duty vehicle fuel economy fell to 25.3 miles per gallon last month from 25.4 mpg in May. US new-vehicle fuel economy reached an all-time high of 25.8 mpg in August 2014. As will remain true hopefully forever, average fuel efficiency in the US is still up – about 20 percent right now – since UMTRI started tracking such figures on October 2007.

New vehicles are having a hard time making headway towards meeting more stringent US emissions and fuel economy goals because of a combination of still-strong truck and SUV sales and lackluster demand for hybrids and plug-in vehicles. For instance, sales of Ford's F-Series pickup trucks jumped 29 percent from a year earlier to almost 71,000 units, while Ford SUV sales increased 7.3 percent from a year earlier. And, while General Motors' Silverado pickup line sales fell 3.7 percent from a year earlier, they still totaled almost 50,000 units, while Chevy Tahoe SUV sales spiked 27 percent.

On the flip side, US sales of hybrids, plug-ins and diesels fell about 21 percent from a year earlier to about 35,000 units. While models like the Chevrolet Volt extended-range plug-in saw higher demand, combined sales of Toyota's four Prius variants fell 27 percent to about 11,000 units, and Nissan Leaf electric-vehicle sales plunged 47 percent from a year earlier to about 1,100 units. Additionally, fleetwide MPG was likely hampered by the lack of high-mpg diesel-vehicle sales from Volkswagen and its Audi division, which have been subject to a stop-sale as a result of VW's diesel-emissions scandal.