Fiat Chrysler Automobiles (NYSE:FCAU) said that its U.S. sales fell 9.7% in September, a drop that it attributed to a planned reduction in sales to rental car fleets. FCA said that its retail sales rose slightly (up 0.3%) from a year ago.

But FCA's small retail sales gain fell well behind rivals' results, despite the fact that the company seems to have the right product mix for the current U.S. market. Why isn't FCA doing better? 

A dark silver Chrysler Pacifica minivan parked in front of a stylish wood-and-concrete building.

Chrysler-brand sales fell 16% in September despite an 18% gain for the handsome Pacifica minivan. Image source: Fiat Chrysler Automobiles N.V.

FCA is losing ground in a market that should play to its strengths

At first glance, FCA's sales decline was a disappointment, an outlier in a month in which overall U.S. light-vehicle sales rose 6.1%. It could have been worse: FCA's result was good enough to beat the average 13% decline expected by Wall Street analysts polled by Bloomberg. 

But it was FCA's 13th straight month of U.S. sales declines, a streak that calls the company's strategy into question. Even FCA's small retail sales gain fell well short of the retail sales increases posted by its key rivals, Ford Motor Company (NYSE:F) and General Motors (NYSE:GM)

A look at the details raises more questions. GM's retail sales rose 8.1% on strong demand for its new crossover SUVs, while Ford's retail sales climbed 4.4% on good results for its Explorer SUV and F-Series pickups. On an overall basis, including fleet sales, GM's sales were up 12% and Ford's were up 8.7%, as both benefited from strong SUV demand and good sales of trucks to commercial fleets. 

FCA has plenty of SUV models as well as a robust line of pickups. And it has been over two years since CFO Richard Palmer said that FCA would work on selling more of its trucks to commercial-fleet buyers. But its pickup sales were down 1% last month, and Jeep sales fell 4%.

What's going wrong? And how worried should FCA shareholders be about these declines?

The real story of FCA's sales decline is in the details

A dive into the details shows that the decline isn't quite as worrisome as the headline numbers suggest. For the most part, FCA's overall sales numbers are suffering as it shuffles and revamps its product portfolio, not because of weaknesses in its recent models. 

For starters, FCA has discontinued several models from a year ago (for good reasons), including the Chrysler 200, Dodge Dart, and Jeep Patriot, and the Dodge Caravan minivan is temporarily out of production pending a redesign. Together, those missing products account for a year-over-year drop of about 20,000 sales, more than enough to explain FCA's total September decline (18,617 fewer sales than September of 2016).

A bright red 2017 Jeep Compass, a compact SUV, on a trail in the woods.

FCA's all-new Jeep Compass is selling very well: Sales were up 75% last month. But its sales gain wasn't quite enough to overcome the loss of the now-discontinued Jeep Patriot. Image source: Fiat Chrysler Automobiles N.V.

Factor them out, and FCA's results are a mix of hits and misses with an overall increase of about 1%. For example, Jeep's overall sales were down 4%, but if we take away the Patriot, the picture changes substantially. Minus the Patriot, Jeep sales rose 11.6% on strong results for the Wrangler, Renegade, Grand Cherokee, and all-new compact Compass. Only the midsize Cherokee, a victim of the rental-fleet cutbacks, was down year over year. 

The Dodge and Chrysler brands were also hurt by missing models, as well as by an industrywide decline in sales of the large cars that now make up disproportionate parts of their portfolios. But the Dodge Durango SUV (up 45%) and Chrysler Pacifica minivan (up 18%) both did quite well in September. As with Jeep, the story for both brands is more positive than the headline numbers would indicate. 

A red 2017 Dodge Charger SRT Hellcat, a full-size sedan, parked on a desert road.

The market has been tough for full-sized sedans, and the mighty Dodge Charger is no exception. Charger sales fell 8% in September. Image source: Fiat Chrysler Automobiles N.V.

Ram sales were flat, and the story here is a little different -- but again, not horrible news. Sales of the two Ram ProMaster commercial vans were up by double-digit percentages, suggesting that FCA might in fact be making some inroads with commercial-fleet buyers. Ram pickup sales were down slightly, but an all-new version of the high-volume Ram 1500 will go into production in January: Buyers may simply be waiting for the new truck. 

The upshot: There are reasons for optimism here

FCA still has work to do, of course. TrueCar estimated FCA's per-vehicle spending on incentives at $4,481 in September, or 13.4% of its average transaction price ($33,404), well above the industry average of 11.5%. Of the major automakers, only GM and Kia Motors spent more as a percentage of their average transaction prices. 

But its latest mass-market products -- the Jeep Compass and Renegade and the Chrysler Pacifica -- are all selling well, as are most of its other SUVs, and its pickups are holding steady in advance of the arrival of a redesigned model in the first quarter of 2018. Long story short, there's enough good news here to justify some optimism.

John Rosevear owns shares of Ford and General Motors. The Motley Fool owns shares of and recommends Ford. The Motley Fool recommends TrueCar. The Motley Fool has a disclosure policy.