Will August 2014 become that proverbial Summer of Love for green-car advocates? Like aging hippies wistfully talking about the great vibes, music and, uh, other stuff emanating from the Summer of '67, those pushing for a cleaner transportation sector can be excused for seeing last August as an apex of sorts. That's because average U.S. fuel economy for new cars has been slowly dropping ever since.

Last month, new-vehicle fuel economy came in at an even 25 miles per gallon, according to a study from Michael Sivak at the University of Michigan Transportation Research Institute (UMTRI). That figure was down 0.2 mpg from September. More importantly, fuel economy has now fallen 0.8 mpg since August 2014.

UMTRI says the culprit is lower gas prices, which have pushed more folks towards SUVs, pick-up trucks and other vehicles of the gas-guzzling variety. Fuel prices are sitting at about $2.22 a gallon, down from $2.29 a month ago and substantially down from $2.96 a year ago, according to AAA.

The good news, in addition to cheaper gas, is that average new-vehicle fuel economy is up about 5 mpg since UMTRI started its current monitoring activity eight years ago. And new-vehicle emissions are about 19 percent lower in the past eight years, though up three percent from that blessed month of August 2014. Additionally, so far, model-year 2015 vehicle's average fuel economy is dead even with the prior year's at 25.3 mpg. Both figures are up from 24.6 mpg for the model-year 2013.

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