General Motors U.S. sales rose 8.9% last month on a daily rate basis, giving the auto maker its best December in five years. But market share fell to an estimated 17.9%, its smallest portion of annual new-vehicle deliveries in the post-World War II era.

GM delivered 245,733 cars and trucks in December, compared with 234,351 year-ago (26 selling days vs. 27 in December 2011). It closed the year with 2.6 million sales in its home market, 3.7% ahead of 2011 but trailing an industry up by double digits and reporting its biggest year-over-year gain since the eve of the Great Recession.

GM’s market share represents a near two-point decline from the 19.7% recorded in 2011, according to WardsAuto data. In the years preceding the economic downturn that helped spin it into bankruptcy, the auto maker routinely would command more than 24% of annual industry sales.

Mark Reuss, president of GM North America, says the market-share loss comes as no surprise given the age of the auto maker’s product portfolio, especially its long-in-the-tooth large pickups and SUVs.

“Our portfolio is the oldest in the industry,” Reuss tells journalists and Wall Street analysts on a conference call to discuss the December results. “Give us 18 months, and you’re going to see the whole portfolio turn.

“It will be the biggest portfolio turn, I think, in U.S. history,” he says.

GM remains the top seller of cars and trucks in the U.S., Reuss notes, adding the company is confident of market-share gains in 2013 as it rolls out such pivotal new models as its redesigned Chevy Silverado and GMC Sierra large pickups.

He says GM’s year-end financial report will demonstrate the auto maker’s progress back from bankruptcy, when in years prior to its Chapter 11 filing it was willing to take share gains at the expense of profits and residual values.

“We’re always concerned about market share, but we’re not going to give (vehicles) away like we did in the past,” Reuss says.

GM did manage to clear some bloated inventory in December, using a combination of discounts and production cuts.

The auto maker’s stock of large pickups fell to 221,649 units, nearly in line with a year-end target of 220,000 and down from 245,853 at the close of November. Silverado transaction prices declined by upwards of $900 from November, but average transaction prices on the truck finished the year higher than for 2011.

Overall, GM’s car and truck inventory closed 2012 at 717,025 units, higher than the range of 650,000-670,000 it sought at the beginning of the year.

“We’re very optimistic about our competitive position and the strength of the industry as we move into 2013,” says Kurt McNeil, GM’s chief U.S. sales analyst.

GM forecasts industry sales in 2013 of between 15.0 million and 15.5 million units, up from an estimated 14.5 million in 2012. If the prediction plays out, the industry will again see its best sales year since the economic downturn.

Looking at specific brands, Chevrolet sales increased 7.7% to 167,091 units with Silverado deliveries up 10.2% to 50,699 after the year-end push.

Sales of the Cruze compact car jumped 32.2% to 21,230 units and demand for the Equinox cross/utility vehicle grew 11.6% to 19,551, helping GM lay claim to being the first auto maker to sell 1 million vehicles achieving at least 30 mpg (7.8 L/100 km) highway in a single year.

McNeil cites increasing availability of advanced-technology powertrains on GM vehicles.

“We’re going to keep investing in these new products and technologies to exceed the increasingly high expectations of our customers,” he says, citing soon-to-launch vehicles as the Chevy Impala large sedan with GM’s eAssist stop/start hybrid system, Buick Encore with its small-displacement, turbocharged 4-cyl. and Cadillac ELR extended-range electric vehicle.

Buick sales rose 14.2% to 16,473 units on the strength of the Enclave large CUV, which saw deliveries increase 13.1% to 6,052 units as customers sought the recently launched redesigned model. GM also moved 4,820 Verano premium compact cars, a 1-year old product that recently added a turbocharged variant.

Cadillac sales improved 16.6% to 18,248 units. The newly launched ATS compact sports sedan accounted for 2,979 sales and the new XTS large sedan sold 2,939.

GMC demand rose 8.7% to 43,921 units, with Sierra deliveries rising 17.8% to 18,710. Sales of the popular Terrain 5-passenger CUV jumped 54.8% to 11,516.