While one fuel-economy researcher says US new-car fuel efficiency fell to a numeric three-year low, fret not, green-car advocates, as it may merely be a case of apples and oranges. On the surface, October's new-car fuel economy dropped 0.4 miles per gallon to 24.8 mpg, University of Michigan Transportation Research Institute's (UMTRI) Michael Sivak and Brandon Schoettle wrote. That's the lowest figure since July 2013.

Or is it? October's statistics factor in calculation adjustments made for 2017 model-year window-sticker values, as UMTRI makes clear. Next month, those calculations will apply for all of the previous figures (UMTRI has been releasing its numbers monthly since October 2007). So, while UMTRI says more light-duty trucks are being purchased each month (isn't that always the case?), that doesn't mean that fuel-economy is going in reverse. Either way, fleetwide MPG is up substantially from the 20.1 average UMTRI first reported nine years ago, so while the numbers aren't exactly sprinting towards that 54.5 mpg Corporate Average Fuel Economy (CAFÉ) earmarked for 2025, which is more like 40 mpg in real-world numbers, automakers are indeed getting batter at cutting their vehicles' collective fuel usage.

Indeed, such a drop in fuel economy is unexpected considering that US green-car purchases appear to be stabilizing, while plug-in vehicle purchases are on the upswing. US purchases of hybrids, plug-ins, and diesels in October were little changed from a year earlier at more than 36,000 units, as a jump in Ford's green-car sales offset the impact of a drop-off in Toyota hybrid sales. Meanwhile, plug-in vehicles sales likely surged by more than 30 percent. So check back with us in a month or so, when UMTRI completes its retroactive calculations.

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