General Motors registers a modest 1.5% uptick in September sales, as the popularity of its passenger cars helps the auto maker stay above water in a surprisingly weak month for truck deliveries.

GM sold 210,245 cars and trucks in the U.S. last month, compared with 207,145 in the same period last year, according to WardsAuto data. There were 25 selling days in September, unchanged from year-ago.

Car sales at the Detroit auto maker grew 29.2% to 88,996 units, but truck deliveries sank 12.3% to 121,249. GM large-pickup sales were off 12.3% year-over-year and large SUV sales plunged 38%.

The downturn in truck demand last month was industrywide, too, with GM, Ford, Chrysler and Toyota combining for a 2.6% shortfall year-over-year in large-pickup sales.

Kurt McNeil, GM’s chief sales analyst, says the seasonal shift toward trucks continues to play out this year as usual, with pickups accounting for an estimated 12.5% of industry sales in September, compared with roughly 10% in the year’s first half.

“GM’s sales decline in this environment is a function of timing more than anything else,” McNeil tells journalists and Wall Street analysts during a conference call to discuss the auto maker’s September performance.

“We began the new model year for the Chevy Silverado and GMC Sierra about 45 days earlier than last year, which prompted many fleet customers to pull ahead their purchases,” he says.

During GM’s August sales conference call, the auto maker said industry truck sales had reached “an inflection point,” where demand would accelerate. It cited big Olympic promotions for a strong truck month in August.

The fullsize Chevrolet Silverado pickup sold 36,425 copies in September, down 16.5% compared with like-2011, and deliveries of its GMC Sierra sibling fell 1.9% to 13,636. Sales of those trucks to fleet customers, however, were down a whopping 55%.

Compared with August, GM’s large pickup sales in September fell 5.4% and large SUV deliveries were flat.

Ed Peper, general manager-fleet and commercial sales, does not expect a repeat performance in October, given the auto maker’s year-to-date pickup deliveries in the segment are up 3% and September’s overall deliveries kept the auto maker on pace for 25% of its sales occurring in the category.

“When we look at the fourth quarter, we see a lot of pent-up demand,” he says. “We anticipate a good final quarter.”

Chevrolet sales last month rose 1.5% to 149,801 units. The bow-tie division saw its greatest strength in cars, where the compact Chevy Cruze sold 25,787 copies to narrowly miss a second straight month of record results by a handful of units.

The midsize Chevy Malibu accounted for 11,188 sales in the month, even with year-ago. McNeil reports 60% of those sales were the redesigned-for-’13 model and just 4,000 units of the previous-generation car remained on dealer lots. Expect advertising for the new model to crank up in the coming weeks, he adds.

Sales of the closely watched Chevy Volt extended-range electric vehicle totaled 2,851 units. It was the niche car’s second straight month of record sales. The new-for-’13 Chevy Sonic compact car delivered 7,525 units.

Sales of the Chevy Traverse large cross/utility vehicle fell 27.8% to 5,687 units.

Buick sales rose 7.9% on volume of 14,673 units. The LaCrosse large sedan posted a 7.7% increase to 4,580 and the all-new Verano small car chipped in 4,042. Sales of the Buick Enclave large CUV fell 13.5% to 4,210 units and deliveries of the Regal midsize sedan plunged 44.7% to 1,839.

Cadillac, which has welcomed the important ATS compact sports sedan and XTS large luxury sedan to dealerships in recent weeks, saw sales tumble 1.3% to 12,579 units. The CTS midsize sedan accounted for 3,103 deliveries, down 33.5%, and sales of the normally rock-solid SRX midsize CUV fell 4.9% to 4,660. Sales of the Escalade fullsize SUV plunged 39.7% to 913.

GMC sales were flat year-over-year, with the Terrain midsize CUV contributing 7,486 deliveries, up 8.3% and good enough to send the vehicle past the 400,000 mark since launching in 2009.

Sales of the Acadia large CUV ran opposite its Traverse and Enclave platform mates, rising 16.5% on volume of 6,159 units. All three of those large SUVs will see redesigned models in showrooms in the coming weeks.

McNeil says GM continues to forecast industry sales of 14.0 million-14.5 million units, with positive economic factors such as rising consumer confidence and a rebounding housing market outweighing headwinds such as Europe’s economic woes, volatile fuel prices and a sluggish job market.

“We continue to be encouraged by positive signs,” he says. “Autos will continue to be a bright spot for the U.S. economy.”

jamend@wardsauto.com