Data compiled by TrueCar shows that, despite gasoline costing only $3.58 a gallon in the U.S., there was still a bit more of a rush to purchase fuel-efficient vehicles in September than there was back in August, when gas averaged a slightly higher $3.65.

TrueCar says that U.S. consumers purchased vehicles that averaged 22.0 miles per gallon in September 2011, compared with 21.7 mpg in August and 21.4 mpg in September 2010. Additionally, the automotive intelligence firm reports that, among individual automakers, Hyundai emerged the fuel economy victor, with an average of 26.7 mpg, while Chrysler's 19.2-mpg average landed it in dead last poition.

Meanwhile, the University of Michigan Transportation Research Institute reports that the average fuel economy of vehicles sold in September came in at 22.1 mpg – unchanged from the University's published month-of-August results. Michael Sivak, a research professor at the University of Michigan's institute, says he's not surprised by the slight discrepancy between his calculations and TrueCar's measure:
The two methodologies and the data sources are slightly different. However, the actual values are not that different from each other
Why does nudging up that 22.0 or 22.1 mpg number matter? Well, TrueCar estimates that an increase of a mere one mpg on the estimated annual U.S. sales of approximately 13 million vehicles would reduce our fuel consumption by 416 million gallons. So, even 0.1 mpg is significant.

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