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GM's pickup sales boomed in 2015, a trend that doesn't look likely to fade any time soon. That's good news for GM's bottom line. Image source: General Motors

General Motors (NYSE:GM) said on Tuesday that its U.S. sales rose 5.7% in December, capping a year in which Detroit's largest automaker saw sales in its home market rise 5%.

That was well short of the roughly 10% gain expected by analysts polled by Bloomberg, and GM shares were down about 3% in mid-day trading on Tuesday.

But GM was quick to point out the silver linings in Tuesday's report, and there were several.

Behind a drop in rental-fleet sales, a solid gain in retail market share
GM's retail deliveries were up 8% in December and 8% for the full year. That was good for a gain in market share of 0.4 percentage points in the United States, a big win for the General.

GM also did well with profitable commercial-fleet sales, up 12% in 2015. Unlike rental-fleet sales, commercial-fleet sales include many high-profit trucks. They're good business, and GM and Ford (NYSE:F) battle hard for customers in this space.

So why were GM's overall gains less impressive? Over the last couple of years, General Motors has been gradually reducing its sales to rental-car fleets. In the past, those sales have accounted for 15% or more of GM's total U.S. sales. That number was down to about 7% in December, or just over 20,000 vehicles, and GM said on Tuesday that the percentage will drop further in 2016.

The reduction makes GM's overall sales gains seem less impressive than they otherwise might be. But from a shareholder's perspective, it's good news. (Read why here.) And there was more good news behind the headline numbers.

Once again, strong sales of pickups and SUVs should power good profits
Like rival Ford, GM posted huge sales of full-size pickups in December. Its Chevrolet Silverado and GMC Sierra combined to sell over 90,000 units in the month, a tremendous number. While Ford's F-Series won the "best-selling model" title once again, GM noted that its share of the retail full-size pickup segment rose more than one full percentage point to 39.2% in 2015. That's big for shareholders: These are very profitable products.

GM's new-for-2015 midsize pickups, the Chevrolet Colorado and GMC Canyon, also had a strong year. The two now have a 32.5% share of the mid-size truck market, up from less than 5% in 2014, GM said. That represents an expansion of the category: Far from declining, the GM twins' biggest-selling midsize rival, Toyota's (NYSE:TM) Tacoma, posted a 16% sales gain in 2015.

GM also did well with crossover SUVs in 2015. The Chevrolet models combined for a 23% year-over-year sales gain, while the small Buick Encore posted a 38% increase.

The upshot: Profitable trends continued for the General
Like many rivals, GM's car sales were mostly not so good in 2015. But again like many of its rivals, the evidence suggests that many of those lost buyers are buying crossovers instead -- and GM is certainly capturing a good share of those buyers.

As shareholders, we like that trend: Generally speaking, crossover SUVs are more profitable than comparable sedan models. Fold in the very strong sales of GM's pickups and big truck-based SUVs, and the stage has been set for a very profitable fourth quarter for GM in North America. We'll find out for sure when GM reports its earnings results later this month.

John Rosevear owns shares of Ford and General Motors. The Motley Fool recommends Ford and General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.