The WardsAuto Fuel Economy Index indicates the average fuel economy of light vehicles sold in the U.S. in May rose to an all-time high of 25.5 mpg (9.2 L/100 km), 3.1% over year-ago and 21.6% above the base index score set in fourth-quarter 2007.

The index rating for May was 1.4% above April’s 25.1 mpg (9.4 L/100 km), the largest month-to-month increase since January 2013. A market shift to smaller vehicles and hybrid and electric powertrains was the primary factor in the boost.

The average for all cars hit a record 29.7 mpg (7.9 L/100 km) last month. Small and midsize cars saw increased market share and record high index scores. Small cars accounted for 19.7% of LV sales and reached 31.0 mpg (7.6 L/100 km). The index rating for midsize cars shot up 2.6% from prior-month to 30.4 mpg (7.7 L/100 km), and market share for the segment rose to 20.1%. Large cars (23 mpg [10.2 L/100 km]) and luxury cars (25.4 mpg [9.3 L/100 km]) each lost market share from April.

Light trucks sold in May scored 21.1 mpg (11.2 L/100 km), matching the 5-month average for this year. Pickups, the lowest-scoring segment at 17.7 mpg (13.3 L/100 km), lost market share to make up 12.4% of indexed sales. On the other hand, CUVs posted a small uptick in market share to 25.9% and regained a record high score of 23.2 mpg (10.1 L/100 km).

Mitsubishi beat Mazda as the top company, hitting 30.2 mpg (7.8 L/100 km). It is the only automaker, excluding electric-only Tesla, to surpass 30 mpg (7.8 L/100 km) in the index history.

Increased sales of small, midsize, and hybrid- and electric-powered cars boosted Nissan to an all-time high 29.2 mpg (8.1 L/100 km). Ford benefited from the same shift in sales and hit a record 24.3 mpg (9.7 L/100 km).

Honda posted a high of 27.0 mpg (8.7 L/110 km) in May. Share of hybrid and electric vehicles fell from April, but consumer preference for small cars outweighed the effect of increased gasoline-powered vehicle sales.

BMW’s score jumped 2.6% from April to 25.2 mpg (9.4 L/100 km), mainly due to increased hybrid sales and the availability of the i3 electric car.

Year-to-date, the WardsAuto FEI averaged 25.2 mpg (9.3 L/100 km), up 2.2% from the same period last year.

eberger@wardsauto.com