The average fuel economy for new cars sold in the U.S. rose to its highest level ever in August, according the WardsAuto Fuel Economy Index, lifting the fuel-economy rating for all light vehicles to its fourth-highest to date.

Light-vehicle sales last month climbed to a seasonally adjusted annual rate of 14.47 million units, the highest monthly SAAR in the past three years.

The fuel-economy improvement was spurred by an increase in small-car sales, which accounted for more than 20% of the market for the first time since March. Large cars made up just 1.38% of LV deliveries, the segment’s lowest share ever.

That change in mix helped push the fuel-economy rating for all cars sold in August to 27.3 mpg (8.6 L/100 km), a thousandth of a point better than the previous record set in March. Also contributing to the record, the luxury-car segment attained its highest-ever FEI rating of 22.8 mpg (10.3 L/100 km), while accounting for 7.2% of the LV market.

Cars and light trucks scored a combined 23.8 mpg (9.9 L/100 km) rating, just 1% less than the record 24.1 mpg (9.7 L/100 km) set in March.

Light trucks outsold cars for the second consecutive month in August, making up 50.05% of LV sales and earning a 20 mpg (11.7 L/100 km) rating, down slightly from July but up 4.2% from year-ago.

Cross/utility vehicles was the best-selling segment for the month, accounting for 24% share, with a 22.2 mpg (10.6 L/100 km) rating.

After losing the No.1 index rating for the first time in 2012 last month, Toyota reclaimed its top spot in August, scoring 27.2 mpg (8.6 L/100 km). Hyundai, with 26.95 mpg (8.7 L/100 km), edged out Kia, with 26.9 mpg (8.8 L/100 km), which was the FEI leader for the first time in July.

Asian auto makers earned a collective 26.1 mpg (9.0 L/100 km) rating in August, while accounting for 45.8% of LV sales.

A boost in Chevrolet deliveries lifted General Motors’ share by 1.3 points from July and helped raise the auto maker’s fuel economy to a company-best 21.9 mpg (10.7 L/100 km).

Chrysler, with 19.8 mpg (11.8 L/100 km), was one of only two auto makers, along with Jaguar Land Rover, with 16.7 mpg (14.1 L/100 km), to earn an index rating of less than 20.

The Detroit Three auto makers, with a 21.3 mpg (11 L/100 km) index rating, combined for 45.4% of the market last month.

European brands accounted for 8.8% of U.S. LV sales in August, with a combined index rating of 23.2 mpg (10.1 L/100 km). Volkswagen finished in the No.4 spot on the index rating with 26.7 mpg (8.8 L/100 km), leading all European auto makers.

The industry year-to-date rating held at 23.8 mpg (9.9 L/100 km), a 4.8% improvement over like-2011 and 13.7% better than the WardsAuto Fuel Economy Index base of 20.9 (11.2 L/100 km) established in fourth-quarter 2007.

jsousanis@wardsauto.com