U.S. new light vehicles achieved record fuel-efficiency for the third month in a row in March, according to the WardsAuto Fuel-Economy Index.

Cars and light trucks sold in the month had a combined 24.1 mpg (9.75 L/100 km) rating, a 1% improvement on the record set in February. It was the first time the index has risen above 24 mpg (9.79 L/100 km).

The new benchmark represents a 15% increase in fuel efficiency over the index’s base rating of 20.9 mpg (11.2 L/100 km), established in fourth-quarter 2007.

March LV sales signified a continued movement toward smaller fuel-efficient vehicles that dominated the first quarter.

Midsize cars, which averaged 27.1 mpg (8.7 L/100 km), comprised 23.5% of all LV sales, an 11-year high that put the segment ahead of Cross Utility vehicles, 22.5 on the index, for the second straight month. CUVs topped the market in the 28 months prior to February.

Small cars enjoyed increased market share as well, accounting for 20.9% of LV sales, while averaging a segment-record 29.7 mpg (7.9 L/100 km).

Light-truck deliveries made up just 46.6% of the market, the vehicle type’s lowest non-Cash-for-Clunkers related share since July 2007.

Changes in the segment mix were accompanied by shifts toward more fuel-efficient vehicles within individual segments.

In addition to the Small Car segment, CUV and Sport Utility Vehicle (17.1 mpg [13.7 L/100 km] segments achieved record index ratings, while Midsize cars enjoyed their second highest rating to date, just below prior month.

Vehicles averaging more than 20 mpg (11.8 L/100 km) accounted for 72.2% of the market, up nearly 50% over the base period, when vehicles getting 20 mpg or more comprised just 48.5% of LV sales.

On the flip side, light vehicles getting 15 mpg (15.6 L/100 km) or less accounted for less than 1% of March deliveries, compared with 3.2% just a year ago and 9.4% in the base period.

While consumer demand, fueled by fear of rising gas prices, would appear to be at the core of the current spike in the index, OEMs played a key role in the overall rating rise.

Auto makers continue to make technological strides in powertrain efficiencies that raise the index rating independent of consumer behavior.

Car companies have rolled out an increasingly large selection of small and midsize vehicles, including a growing number of hybrid and alternate-power vehicles that provide consumers interested in fuel economy greater choice than ever.

Indeed, vehicles rated higher than 30 mpg (7.8 L/100 km) on the index accounted for 11.8% of sales in March, up from 4.3% year-ago. The 270% increase was made possible, in part, by the increasing number of vehicles available in that category.

The market also likely is seeing a bounce in demand of small and midsize vehicles that had been in short supply for most of 2011, after Japan’s March earthquake and tsunami disrupted global production.

Whether the bump is sustained or not, the current rise of more fuel-efficient vehicles is accompanied by a shift in share between auto makers.

General Motors, while earning its best index rating to date of 21.4 mpg (11 L/100 km), saw sales dip for some of its less fuel-efficient vehicles, earning the company its smallest share (16.5%) of the U.S. market ever.

Toyota set a new index rating record for a single company of 28.3 mpg (8.3 L/100 km), while earning its highest share in 15 months and selling 200,000 vehicles for the first time since August 2009.

Honda may have experienced a short-term supply issue last month, but Nissan at 25 mpg (9.4 L/100 km) saw its highest volume month ever. The auto maker accounted for 9.74% of all March LV sales, 1/100th of a share point less than its best-ever share.

Hyundai took the No.2 index spot in March with a 27.2 mpg (8.6 L/100 km) rating followed by Kia with 27.1 mpg (8.7 L/100 km) and Volkswagen with 26.9 mpg (8.7 L/100 km), both of which established company records.

Domestic-built light vehicles broke February’s previous all-time high with an average score of 23.1 mpg (10.2 L/100 km) but lost more ground to imported cars and trucks, which collectively scored 27.2 mpg (8.6 L/100 km), also a record.

The year-to-date WardsAuto Fuel Economy Index rating for the industry rose to 23.9 mpg (9.8 L/100 km), marking a 4.5% gain on first-quarter 2011 and the single-highest quarterly rating to date.

jsousanis@wardsauto.com