Maybe auto sales are cooling off after all: General Motors (NYSE: GM), Chrysler, and Toyota (NYSE: TM) all reported lower gains than seen recently -- in fact, GM reported a decline -- as fleet deliveries slowed and some categories of vehicles, such as GM's pickups, drew notably fewer buyers than in recent months.

At Ford (NYSE: F), which reported a worse-than-expected 4% drop in sales in July versus year-ago totals, the news was something of a mixed bag.

On the retail side, some encouraging numbers
While Ford's PR team attributed its drop largely to a decline in fleet sales, its retail sales -- which were up 2% -- were also sluggish relative to recent overall market trends. That's not new -- Ford's 2012 sales through June were up 6.6%, versus 14.2% for the overall market -- but it continues to be a point of concern.

But there were bright points for Ford in July. The outgoing Fusion, which is in "sell-down" mode in advance of its successor's arrival this fall, posted its best July result ever. Sales of the white-hot Explorer SUV continued to be strong. And -- unlike at GM, where big pickups posted a double-digit drop -- Ford's full-sized pickup, the F-series family, posted a small gain over strong year-ago results, continuing a recent streak of increases.

The F-series is Ford's cash cow -- it's a high-profile, high-margin product that also happens to be the single best-selling vehicle in America. Ford has invested heavily to maintain its lead, and one of its key selling points recently has been the fuel-efficient "EcoBoost" drivetrain offered as an option on the big pickups.

EcoBoost is Ford's name for a line of innovative turbocharged engines. The V6 variant offered in the F-series offers V8-like power with improved fuel economy. It remains very popular with cost-conscious buyers, accounting for 42% of F-series sales in July.

The continued strength of Ford's big pickup -- with "pickup season," traditionally the second half of the year, still mostly to come -- bodes well for the Blue Oval even in a sluggish overall market. But there were some notes of concern. Sales of the compact Focus were up over year-ago numbers, but the sales pace was a bit off that of recent months -- though production constraints may be a factor. And sales of all of Ford's SUVs other than the Explorer were down -- including sales of the recently white-hot Escape.

Ford officials pointed out that the Escape is in transition, with dealers selling off the very last of the old 2012 models while still building up inventories of the all-new 2013 version. Ford Vice President Ken Czubay did say on Wednesday that the 2013 Escape has the highest "demand to inventory" of any Ford product right now. That's another sign that the new SUV is likely to see good success in the coming months.

But the drop in fleet sales was notable.

A steady market for Ford starts to slip
The Blue Oval's sales analyst, Erich Merkle, noted on Wednesday that fleet sales represented just 27% of Ford's total in July, down from 31% last year. To date in 2012, fleet sales have represented about a third of Ford's total sales, so July represents a significant departure from recent trends.

By way of background, there are three basic categories of fleet sales: "commercial" sales (think big contractors buying a new fleet of pickups, or a business buying company cars for its salesforce), "government" sales (think police cars and municipal trucks), and "daily rental" sales (rental cars). For Ford in July, commercial sales held steady, while government and rental sales both dropped.

What's behind those drops? According to Merkle, there are a couple of factors at work. Sales to governments are off because state and local budgets are constrained: Cities and towns are keeping their police cars in service longer and postponing other purchases, because they're strapped for cash. Rental-car sales -- generally thought to be the least desirable kind of fleet sales -- were down largely because of a seasonal ebb and flow of orders.

Merkle did make a point of saying that Ford has been "disciplined" in keeping daily rental sales at a modest percentage of overall fleet sales. According to Merkle, industrywide sales to rental fleets have been up roughly 15%, while Ford's are up just 1%. While Ford thinks some rental-car sales are a good thing -- among other benefits, having cars in rental fleets exposes them to potential buyers who might not otherwise consider a Ford -- too many can erode margins and retail resale values.

The upshot: concern, not worry
Ford's execution isn't a point of concern. The company's recent models have been outstanding, and all signs suggest that they are finding plenty of eager buyers. And Ford's inventories remain at a reasonable 60 days, Merkle said on Wednesday, so production remains in line with demand. But the Blue Oval's U.S. results -- the driver of its profits in North America -- have been far and away the brightest spot in the company's so-so recent financial results, and that leaves it vulnerable to a downturn at home.

Given the severe economic challenges that have led to big losses in Europe, and Ford's ongoing heavy investments in Asia, the company's profits will probably rise and fall with the U.S. market for at least the next few quarters. The good news is that Ford is making the best of the situation, with strong products, appealing and profitable options packages, and much-reduced incentives.

But the not-so-good news is that, if U.S. auto sales really start to slip, so will Ford's profits. Watch this space.

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