After many years of strong growth following the Great Recession, the U.S. auto market started to peak last year. This has led to more volatility in automakers' results from month to month, rather than the steady growth that was typical a few years ago.

This volatility was evident in March. General Motors (NYSE:GM) continued to gain market share, whereas Ford Motor (NYSE:F) and Fiat Chrysler (NYSE:FCAU) both posted sales declines. However, Ford and Fiat Chrysler performed well in the lucrative full-size pickup segment -- just where GM was at its weakest.

Sales rise at GM -- with one notable exception

General Motors delivered 256,224 vehicles in the U.S. last month, up 2% year over year. Retail sales rose at an even faster 5% clip, offset by another planned decline in less-profitable sales to rental car agencies. GM's retail market share increased by 0.6 percentage points to 16.1% in March, based on the company's internal estimates.

While GM posted solid sales numbers overall, it was surprisingly weak in the pickup market. Deliveries of the Chevy Silverado full-size truck declined 11.6% to 42,410 units. Its corporate sibling, the GMC Sierra, posted an even bigger 14.3% decline, with 18,460 deliveries.

A Chevy Silverado pickup truck

Chevy Silverado sales continued to slide in March. Image source: General Motors.

This pocket of weakness extended to the midsize truck market. The Chevy Colorado and GMC Canyon both posted double-digit percentage sales declines last month, after seeing big gains in recent years. In total, GM's pickup truck deliveries declined by more than 10,000 units year over year last month.

Ford and Fiat Chrysler underperform -- but not in the truck market

Ford and Fiat Chrysler both trailed GM in terms of sales performance during March. Total deliveries were down 7% year over year at Ford and down 5% at Fiat Chrysler. They, too, are deliberately reducing deliveries to rental car companies, which had an impact on their sales numbers. That said, Ford and Fiat Chrysler posted declines even on a retail-only basis, with deliveries down 1% to 2% year over year for both companies.

Their sales weakness did not extend to the pickup market, though. Ford's deliveries of F-Series trucks surged 10% to 81,330 units last month -- exceeding General Motors' sales across all of its pickup models combined.

Fiat Chrysler also gained ground last month, though not to the same extent as Ford. Ram pickup deliveries increased 6% in March to 46,384. That was enough for Ram to overtake the Chevy Silverado as the No. 2 pickup. (That said, including the GMC Sierra, GM retained a comfortable lead over Fiat Chrysler in the full-size pickup market.)

A trend is forming in the truck market

GM's underperformance in the pickup market is a theme that has been building up over the past year. On the whole, GM has been gaining market share in the U.S. thanks to a slew of relatively new models, but it is starting to lose share in the full-size truck segment.

Obviously, it's not ideal from investors' perspective that the General's weak spot is one of the most profitable segments of the auto market. Yet this is just a natural consequence of having an older truck lineup than its top rival. The current-generation Chevy Silverado and GMC Sierra started shipping in the first half of 2013, whereas the Ford F-150 was redesigned for the 2015 model year.

A Ford F-150 pickup truck

The 2015 Ford F-150 was an all-new design. Image source: Ford Motor Company.

Help is coming, though. GM will launch "all-new" versions of the Chevy Silverado and GMC Sierra in 2018 as 2019 model year vehicles. The new trucks will incorporate more aluminum, copying the shift toward using more of the lightweight metal Ford began in 2015 with its F-150s.

Furthermore, General Motors has logged strong sales in other lucrative segments of the auto market lately. Most notably, it posted double-digit percentage growth in full-size SUV deliveries during Q1. Van sales have soared as well, with deliveries up more than 25% year-to-date.

Ford and Fiat Chrysler are making gains in the full-size truck market, but they are struggling to grow otherwise. Meanwhile, GM is benefiting from its fresher product portfolio in most market segments. The full-size truck segment is a big exception for now -- but with brand-new pickups coming at GM by the end of 2018, this won't be a problem for very long.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.