The small- and middle-car shortages that have plagued U.S. sales in recent months contributed to a year-over-year drop in the U.S. average fuel economy of new light vehicles, according to a WardsAuto report.

LVs sold between July and September averaged 22.2 mpg (10.7 L/100 km), down from the prior quarter’s 22.5 mpg (10.4 L/100 km). It was the second consecutive quarterly drop in WardsAuto’s Fuel Economy Index rating.

The decline in fuel economy came in the context of extremely low inventories, particularly of small cars, as a result of the disruption to Japanese production caused by the March 11 tsunami and earthquake.

Monthly small-car inventory averaged 229,250 units during the third quarter, a 38.5% drop vs. year-ago. Honda and Toyota, longtime leaders in the segment, operated with small-car stocks that were down an average 67% and 64%, respectively.

As other auto makers, including Ford (-41%) and Hyundai (-74%) also dealt with decreased small-car inventories, the segment accounted for just 17.4% of quarterly LV sales – down from 20.1% in the prior quarter.

Total car stocks and sales fell in the quarter, with passenger cars accounting for 46.3% of LV sales in the period, the lowest third-quarter car share since 2005.

Volkswagen led the industry with a 25.6 rating on the WardsAuto index. Toyota was in the No.2 spot with a 25.1, followed by Nissan at 24.9 and Kia with 24.8.

General Motors, whose Chevy Cruze was the top-selling small car for the second straight quarter, led Detroit auto makers with a 20.7 index rating, followed by Ford at 20.1 and Chrysler with 19.1.

Related document: Ward’s Fuel Economy Index

The overall fuel-economy rating for cars irrespective of segment was 25.5 mpg (9.2 L/100 km), up 1% from year-ago’s 25.3 mpg (9.3 L/100 km), but the effect was offset by the vehicle types reduced market share.

The light-truck index rating of 19.1 was even with prior-year and down slightly from the second quarter’s 19.3, due to an increase in market share for both pickups and SUVs.

Domestic LVs in the third-quarter slid from 21.8 in the second quarter to 21.4 on the latest FEI, down 1.3% from year-ago. Domestic cars earned a 25.1 rating, while light trucks registered 18.6.

Imported LVs finished even with year-ago, despite a 0.2% quarter-to-quarter falloff equating to a 24.9 mpg (9.4 L/100 km) rating with cars at 26.4 mpg (8.9 L/100 km) and light trucks at 22 mpg (10.7 L/100 km).

Despite the recent fall in quarterly fuel efficiency, the ’11 model-year (October 2010 through September 2011) finished with an overall rating of 22.4 mpg (10.5 L/100 km) a 1% increase over ’10 model-year results.