The average fuel economy rating of new light vehicles sold in the U.S. in November was 24.8 mpg (9.5 L/100 km), according to the WardsAuto Fuel Economy Index.

Year-over-year improvements lessened for six months and finally resulted in the first decline in over four years. The November 2014 result was 0.1% below same-month 2013.

The national average gasoline price was $2.997 a gallon last month, the first time below $3 since November 2010. The figure was 9.8% less than the same month last year, resulting in the biggest year-over-year price drop in 12 months. A 7.9% decrease from October was the greatest month-to-month decline since December 2008, when prices fell 21% from that November.

Hybrid and plug-in hybrid powertrains lost market share in November in favor of conventional gasoline-powered vehicles.

Nation-wide diesel prices fell to the lowest point since February 2011, averaging $3.647 a gallon in November 2014. This fuel price also has shown a downward trend, though not as strong as gasoline. Last month’s figure was 0.9% below the previous month and down 5.0% from same-month 2013. The November share of diesel-powered vehicles grew to 2.5% from 2.2% last year.

As updated model-year 2015 vehicles continue to saturate the market, the overall index score is expected to rise, but decreasing fuel prices have increased consumer interest in less fuel-efficient models.

The November index rating of cars sold (29.1 mpg [8.1 L/100 km]) rose 0.6% from last year, but market share fell 2.4 percentage points. The score for midsize cars showed a 1.0% decline, and large cars posted a 1.5% dip from 2013.

Light trucks (20.9 mpg [11.2 L/100 km]) showed a 0.9% rating improvement, while market share grew from 53.0% in 2013 to 55.4% this year.

The most improved segment in November was vans, up 1.9% to a record-high 21.0 mpg (11.2 L/100 km). However, vans were the only light-truck segment to lose market share, counteracting any positive effect on the LV average.

Mitsubishi (29.5 mpg [8.0 L/100 km]) remained the highest-rated automaker (excluding electric-only Tesla) and was one of the few automakers without a shift to larger vehicles.

BMW showed the biggest year-over-year gain, up 12.4% with the help of the new i3 electric vehicle and i8 plug-in hybrid along with a doubling in its share of diesel models.

Through the first 11 months of this year, the WardsAuto Fuel Economy Index stood at 25.1 mpg (9.4 L/100 km), a 1.8% improvement from the same period of 2013.