The WardsAuto Fuel Economy Index indicates the average fuel economy of light vehicles sold in the U.S. in September rose 1.5% over year-ago and 16% above the base index score set in fourth-quarter 2007.

Combined fuel economy for all LV sales in the month was 24.3 mpg (9.7 L/100 km), a 1% decline from August’s record 24.6 mpg (9.6 L/100 km) rating and the first month-to-month decline since May 2012.

September was the third consecutive month national average gasoline prices decreased, presumably lowering consumer interest in the most fuel-efficient powertrains. The share of gasoline-powered models increased, after shrinking in the previous two months. Concurrently, the share of hybrid-electric and plug-in hybrid-electric vehicles fell.

Furthering the index decrease was a drop in the market share of small and midsize cars and cross/utility vehicles (the most fuel-efficient vehicles of their respective vehicle types) from the previous month.

Labor Day weekend sales at the end of August incentivized consumer interest in cheaper, smaller vehicles, likely pulling ahead sales of those vehicles from September.

Volumes of larger and more expensive segments targeted at less price-sensitive consumers held steady from August to September.

The average fuel economy for all cars in September was 28.1 mpg (8.4 L/100 km), 2.1% higher than like-2012 but 1.2% lower than August. The downward pull was generated primarily from a 1.4% month-to-month FEI drop in midsize cars to 28.2 mpg (8.3 L/100 km).

The light-truck FEI rating showed a 1% improvement from last year to 20.3 mpg (11.6 L/100 km). Trucks had a smaller down-tick from the previous month, 0.2%.

Volkswagen remained the top automaker (excluding electric-only Tesla) for the fourth consecutive month with a 27.8 mpg (8.5 L/100 km) FEI score. However, VW’s score was down from prior-month, affected by the market trend of larger vehicles and less fuel-efficient powerplants.

While the overall market rating was down from August, Mazda, Nissan and Daimler saw record highs in September. Mazda (26.6 mpg [8.8 L/100 km]) was one of the few companies to have a favorable shift to small and medium cars. Nissan (26.4 mpg [8.9 L/100 km]) had significant gains in hybrid and electric vehicles for both its Nissan and Infiniti brands. Daimler’s Mercedes-Benz was the most-improved brand (20.9 mpg [11.2 L/100 km]), boosted by a preference for cars over light trucks and diesel engines over gasoline.

Year-to-date, the WardsAuto FEI was 24.4 mpg (9.7 L/100 km), up 1.9% from the same period last year.

eberger@wardsauto.com