New vehicles sold in the month of September averaged 25.3 miles per gallon, a drop of 0.5 mpg from the August rating, which was the highest fuel economy rating ever for new cars. Compared with October 2007, fuel economy ratings on new cars sold has improved by 5.2 miles per gallon, or nearly 26%.

The data are based on the average sales-weighted fuel economy rating printed on a new car’s window sticker and are compiled by Michael Sivak and Brandon Schoettle of the University of Michigan Transportation Research Institute.

The sales-weighted unadjusted Corporate Average Fuel Economy (CAFE) performance rating averaged 30.7 miles per gallon in September, up 6 mpg since October 2007. These values are not directly comparable to the window-sticker ratings because these are adjusted by the EPA and used to derive the window-sticker ratings.

Booming sales of SUVs and light trucks as gasoline prices fall are likely the culprit in the lower fuel-economy rating in September. According to Kelley Blue Book, sales of full-size SUVs and crossovers is up nearly 7% year-over-year for the first nine months of the year and rose 14% in September, compared with September 2013. Sales of full-size pickups were up 5.2% in the first nine months of the year and up 19% compared with last September. Mid-size SUV and crossover sales rose even faster: 11.5% for the year to date and nearly 20% over last September.

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