The average fuel economy of new cars sold in the U.S. in November dropped by 0.1 mile per gallon, continuing a trend that began in August of last year when pump prices began to fall, according to new data by the University of Michigan Transportation Research Institute. New cars sold in November got an average of 25 miles per gallon , down 0.8 from the peak reached in August of 2014, but still up from 4.9 mpg since October of 2007, when the university began tracking sales.