On Thursday, Ford Motor (NYSE:F) reported another month of solid sales gains in the U.S. pickup market. The company sold 69,651 F-Series trucks in September, up 16.4% year over year. F-Series retail sales surged an even more impressive 28%. For the full third quarter, F-Series sales reached 207,271: Ford's best sales performance since 2006.

The 2015 Ford F-150 had another good month in September. Image source: Ford Motor Company.

Despite posting this strong growth, Ford came in second to General Motors (NYSE:GM) in the full-size truck market yet again. With supply constraints for the popular Ford F-150 easing, it's harder to dismiss GM's sales lead now. Moreover, with GM refreshing the Chevy Silverado and GMC Sierra full-size pickups for the 2016 model year, it won't be any easier for Ford to gain ground next year.

Ford loses its sales lead
Prior to the launch of the innovative 2015 Ford F-150 late last year, Ford was the clear market leader in the U.S. full-size truck market.

In 2012, Ford sold 645,316 F-Series trucks in the U.S., while combined sales across GM's two full-size truck models (the Chevy Silverado and GMC Sierra) totaled 575,497 units. That gave Ford a 12% advantage in unit sales. In 2013, GM's U.S. truck sales grew to 664,803 units, but Ford sold 763,402 F-Series trucks, widening its advantage to 15%. (In both years, Fiat Chrysler was a distant third, selling less than half as many pickups as Ford.)

Through the first few months of 2014, Ford maintained its double-digit sales lead over GM. But as it neared the transition to the 2015 F-150, Ford's truck sales slowed. To some extent, that was on purpose, as Ford wanted to build up its inventory to offset the months of plant downtime needed to support the changeover.

Ford dealers have been short on F-150s for most of 2015.

In any case, the result was that Ford F-Series sales declined 1.3% in 2014, while GM continued to post double-digit growth in full-size truck sales. This narrowed Ford's lead to less than 2% for the full year.

That remaining advantage has vanished this year. In the first half of 2015, Ford faced pretty significant supply constraints with only one of the two Ford F-150 plants operating. As a result, GM has sold 602,557 full-size trucks through September, nearly 7% ahead of Ford's 564,451 full-size truck sales.

GM's accomplishment is even more notable because the company reentered the mid-size truck market late last year. Many analysts expected this to cannibalize its full-size truck sales. Instead, GM has posted double-digit growth in Silverado and Sierra sales while also selling more than 85,000 mid-size trucks year-to-date.

Supply constraints easing
Ford's F-150 supply constraints are finally easing. In a conference call on Thursday morning, the company noted that while it sold nearly 70,000 F-Series trucks in September, it was still able to increase its inventory by about 7,000-8,000 units. This also allowed Ford to increase fleet sales to 18% of total F-Series deliveries -- just a little shy of its historical average.

Even so, Ford still trailed GM in full-size truck sales last month. Full-size truck sales rose to 73,479 in September at GM. Including the mid-size Chevy Colorado and GMC Canyon, its total truck sales exceeded 83,000 units.

Did Ford miss the market?
Ford could still make up some more ground as inventory improves, particularly by meeting pent-up demand from commercial customers. Yet it seems like the days of Ford holding a double-digit lead in the full-size truck market are over. Just getting back to parity with GM is starting to seem like the best Ford can hope for.

Ford invested a huge amount of money moving to a lightweight aluminum body for the 2015 Ford F-150. This contributed to significant fuel economy gains. Unfortunately, the truck became available just as gas prices were crashing toward the $2/gallon level.

The 2015 Ford F-150 remains a very impressive truck and Ford has achieved big gains in its average transaction price this year, offsetting the higher cost of building the new model. That said, the company appears to have missed out on sales growth opportunities. Some buyers don't seem to be willing to pay for the 2015 F-150's improvements, perhaps because fuel prices have fallen significantly in the past year.

Whatever the reason, General Motors has become king of the hill in the lucrative full-size truck market. Ford has its work cut out for it in order to regain the crown.

Adam Levine-Weinberg owns shares of 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.