After declining for several months in a row, the average sales-weighted fuel economy of new vehicles sold dropped to its lowest figure in over three years.

Calculated from the monthly sales of individual models of light-duty vehicles and the combined city/highway fuel economy ratings published in the EPA Fuel Economy Guide, October saw the sales-weighted fuel economy drop to 24.8 mpg, the lowest since July 2013. It’s worth noting, however, that starting with model year 2017, the Environmental Protection Agency (EPA) has implemented a new method for calculating the window sticker fuel economy for new light-duty vehicles, having an effect on the average fuel economy figure.

With those changes, the EPA has also adjusted the window sticker fuel economy for some model year 2011-2016 vehicles using the new method. University of Michigan’s Transportation Research institute researchers Michael Sivak and Brandon Schoettle said next month’s update will incorporate the retroactive calculation adjustments for previous model years.

It’s not likely that the average fuel economy of new vehicles sold really dropped significantly, but rather it’s no longer a fair comparison with EPA’s new method for calculating window sticker fuel economy on new vehicles. Expect a clearer picture next month when the researchers announce their November data after completing its retroactive calculations.

This article originally appeared at AutoGuide.com