Last month, that figure dropped 0.1 miles per gallon. That means that the average car in the US for June 2015 was 25.4 mpg. The highest ever recorded was 25.9 mpg, recorded in August 2014. The good news is that the overall fuel economy is up 5.3 mpg since the time when UMTRI started looking at this number, back in October 2007. An email sent by UMTRI's Michael Sivak speculated that the 0.1-mpg dip from May, "likely reflects the increased sales of light trucks and SUVs in June."
What does this all mean? Well, UMTRI has a number to help here, too. It's called the University of Michigan Eco-Driving Index (EDI) and it "estimates the average monthly emissions of greenhouse gases generated by an individual US driver." The vehicles that Americans bought in April will emit, on average, nine percent more GHG than the average was last August, but we're still doing 15 percent better than we were in October 2007. You can get more details from UMTRI here.