General Motors says it expects sales of its all-new large pickups to gain momentum this month as the mix of powertrain and body-style options improves.

The lack of key inventory, combined with aggressive incentive spending by rivals, helped send the auto maker’s U.S. deliveries down 3.2% in September.

Kurt McNeil, GM’s chief sales analyst, says fewer selling days in September compared with the same period year-ago, as well as an anticipated slide in fleet sales, also contributed to the lackluster results.

GM sold 187,195 light vehicles last month over 23 selling days, compared with 210,254 deliveries over 25 days in like-2012, according to WardsAuto data. So far this year, the automaker’s sales are up 7.6% and executives see a brisk close to 2013.

 “As we look toward the fourth quarter, we expect that car-buying fundamentals will remain strong,” McNeil says in a conference call today to discuss September’s results.

McNeil cites a recent drop in unemployment claims signaling a coming acceleration in new jobs, as well as friendly monetary policies such as low interest rates, a recovering housing market, stable gasoline prices and rising household wealth.

“It is clear we should be in good shape going forward,” he says.

As for the federal government’s shutdown due to a budget impasse underpinned by opposition to President Obama’s new health-care law, GM sees no reason to panic.

“We have a lot of strong fundamentals,” says Sue Yingzi Su, senior economist-North America. “We should be able to weather the turbulence at this time.”

A storm of incentives from competitors, however, dampened sales of its important large pickups last month, the automaker says.

Sales of the Chevrolet Silverado and GMC Sierra typically account for a good chunk of GM’s monthly volume, and in September deliveries of the pickups were flat at 45,944 units. The performance comes as GM launches redesigned-for-’14 models and Ford begins selling down ’13 F-150s to make room for a redesigned model coming next year. F-150 sales in the month grew 20.7% to 57,289 units.

GM claims discounting of more than $5,000 per unit on the F-150 in September played a role. GM’s pickup incentives in the month were the lowest in the industry, McNeil says, and lower compared with August.

“It’s clear we took a disciplined approach to the marketplace,” McNeil says, declining to comment on how long Ford might continue its strategy. “We’re going to work our plan. We’re not going to give up share, but at the same time, as we are introducing new products, we’re not going to get off our game.”

Don Johnson, head of U.S. sales for Chevrolet, says GM fullsize pickup deliveries should gain momentum in the coming weeks as the mix of powertrain and body-style options improves. He says a report of a shortage of the volume 5.3L V-8 engine has not affected sales.

“We’re into the heart of our launch in October,” he says. “We’ve got the strongest trucks out there. Once we get our cab mix in line, we’re ready to take on all comers.”

A 27% decline in fleet sales also affected GM’s results last month. The automaker blames timing of customer deliveries with some buyers taking units earlier than usual, in addition to discontinued sales of the Chevy Colorado and GMC Canyon midsize pickups and a greater focus on retail sales of the Chevy Impala large sedan with its redesign.

“A lot of things happen when customers want vehicles in certain months, and this was a classic case,” says Ed Peper, who heads sales of GM vehicles to rental-car companies and government and commercial buyers.

Standout vehicles for GM in the month included the all-new Buick Encore small cross/utility vehicle, which recorded 3,206 deliveries in September. The Cadillac ATS, also an all-new product, saw 2,739 deliveries in the month. The Chevy Equinox and GMC Terrain midsize CUVs combined for 22,584 purchases.

The Chevy Cruze compact sedan, however, witnessed a 46.3% sales swoon to 12,730 units, blamed primarily on lower fleet sales and tight inventories at the beginning of the month.

jamend@wardsauto.com