December’s month-over-month uptick in U.S. light-vehicle sales corresponded with a decline in average fuel economy, as more customers bought larger vehicles in the last month of the year.

Light trucks achieved their highest monthly market share, 50.1%, since January 2009, helping drop the Ward’s Fuel Economy Index rating to 21.9 mpg (10.7 L/100 km) from 22.0 in November.

However, powertrain advancements, diminished annual truck sales and above-trend fuel-economy spikes in July and August from the “Cash for Clunkers” rebate, spurred an upsurge. The overall fuel economy of an average new light vehicle in the U.S. rose 3.3% last year to 22.2 mpg (10.6 L/100 km), compared with 2008’s 21.5 mpg (10.9 L/100 km).

Related document: Ward’s Fuel Economy Index

The year-over-year efficiency increase is the cumulative effect of several consumer buying trends. Light trucks accounted for the smallest share of the light-vehicle market, 47.5%, since 1997.

And as cross/utility vehicles claimed a larger share of light-truck sales at the expense of less-efficient large pickups and SUVs, the overall rating for light trucks rose 4.1% to 18.7, up from prior-year’s 17.9 rating.

Car sales saw both Small Car and Middle Car groups increase their market share while Large and Luxury Cars declined, according to Ward’s segmentation. As a result, the overall passenger-car rating finished at 25.3 mpg for 2009, up 2.4% from 2008.

Also playing a role in the upward movement of Ward’s FEI was the increased share of hybrid vehicles, which rose 16.6% in 2009 to 2.8% of the light-vehicle market, compared with 2008.

This year, the industry will be hard-pressed to equal the ratings achieved in July and August under the auspices of the Cash for Clunkers program. But it’s also possible the general buying trends that have emerged during the current recession will reverse, or at least slow, as the economy recovers, limiting the potential rate of increase in average fuel efficiency.

A turnaround in the housing and construction sectors almost undoubtedly lead to a rebound in large pickup share, while any economic upticks that increase personal earnings or expand the availability of financing could slow the move toward smaller, less-expensive and generally more-efficient cars.

Auto makers have pinned much of their hopes for the coming model years on the success of new smaller vehicles, with increased offerings in the B-Car segment one of the more significant characteristics of upcoming fleets.

Toyota Motor Sales U.S.A. continued its reign atop Ward’s FEI in 2009, achieving a 26.0 rating, up 4.6% from its industry-leading 24.8 rating in 2008.

Hyundai Motor Corp. and American Honda Motor Co. Inc. were next on the index, both earning a 24.2 rating.

Mitsubishi Motors Corp. was the most-improved auto maker on the index, increasing its rating 6.9% to 22.6 and moving from 10th position on the list to 9th.

Tata Motors Ltd., home to the Jaguar and Land Rover brands, was the second-biggest gainer of the year, increasing its index rating 6.3%. However, its 15.8 rating still was the lowest among auto makers tracked by Ward’s.

General Motors Co. led Detroit-based auto makers with a 2009 rating of 20.2, up 0.9% from the prior year. Ford Motor Co. closed the gap, rising 4.1% from a 2008 rating of 19.2 to a 20 mpg rating for total 2009.

Chrysler Group LLC, meanwhile, was the only car company whose index rating fell in 2009, as light trucks accounted for 75.4% of the beleaguered auto makers’ sales – up from prior-year’s 72% truck share. Its 18.6 rating last year left it at the bottom of all non-luxury auto makers.

Ward’s FEI is calculated using the Environmental Protection Agency’s fuel-economy rating for individual models and engine-installation rates gathered by Ward’s to estimate the relative fuel efficiency of U.S. car and light-truck sales on a monthly basis.

jsousanis@wardsauto.com