In the first half of 2019, Ford Motor Company (NYSE:F) maintained its dominance of the domestic truck market. Meanwhile, Fiat Chrysler's (NYSE:FCAU) Ram brand made extensive market share gains at the expense of the industry's traditional No. 2 player, General Motors (NYSE:GM).

Things got even more interesting in the third quarter. This time around, it was GM's turn to assert itself -- at the expense of Ford. Fiat Chrysler continued its strong growth in the truck market, as well. As a result, the market share battle in the domestic truck industry is becoming more closely contested than ever before.

GM makes its move

Domestic deliveries of the Chevy Silverado -- GM's main pickup model -- fell 12% in the first half of 2019 to 256,777 units. Including the Silverado's more expensive cousin, the GMC Sierra, GM's total full-size truck sales fell nearly 10% to 354,180 units. Sales of the top U.S. automaker's two midsize pickups, the Chevy Colorado and GMC Canyon, slipped by a combined 3% during the same period.

Rising competition, mainly from the reintroduction of the Ford Ranger earlier this year, likely drove the decline in midsize truck sales. By contrast, GM blamed the drop in full-size truck sales on inventory constraints related to the introduction of all-new light-duty truck models for the 2019 model year. Management indicated that the changeover would no longer be a headwind in the second half of the year.

Sure enough, sales of General Motors' full-size trucks rebounded last quarter. GM and its dealers delivered 155,482 Chevy Silverados in the third quarter, up 17% year over year. GMC Sierra sales soared 29% to 66,198 units. In total, GM delivered 221,680 full-size trucks in the quarter, compared to 184,697 in the prior-year period -- a 20% year-over-year increase.

A white Chevy Silverado truck driving on a rural road.

Chevy Silverado sales bounced back in Q3 after two weak quarters to start 2019. Image source: General Motors.

The General's midsize pickups didn't fare as well in Q3. Deliveries of the Colorado and Canyon combined fell 10% to 39,094 units. Still, those models made a meaningful contribution to GM's overall truck business, and GM's total truck deliveries still rose 14% year over year to 260,774 units last quarter.

Ford full-size truck sales slip, but the Ranger keeps rising

For the second consecutive quarter, Ford's full-size truck sales fell last quarter. Deliveries of its best-selling F-Series trucks fell 6% year over year to 214,176 units. Adding in the Blue Oval's heavy truck franchise, full-size truck sales declined 5% to 218,421 units.

While F-Series sales declined, the relaunched Ford Ranger midsize truck continued to gain traction. Ford and its dealers delivered 26,211 Rangers last quarter, nearly as many as in the first two quarters of the year combined.

Including the Ranger, the F-Series lineup, and heavy trucks, Ford's total truck deliveries rose 6% year over year to 244,632 units in the third quarter. That's an impressive result, particularly because the Ford F-150 is competing against much newer models from GM and Fiat Chrysler. Still, whereas Ford easily beat General Motors in terms of total domestic truck deliveries in the first half of 2019, GM retook the truck sales crown in Q3.

Fiat Chrysler's market share gains continue

Longtime No. 3 Fiat Chrysler achieved further share gains in the truck market last quarter, albeit more at the expense of Ford (and smaller players in the truck market) than in the first half, when GM was the big loser. First off, Ram pickup sales surged 14% to 161,635 units -- although that did represent a slowdown from the 28% growth rate logged in the first half of the year.

Second, sales of the new Jeep Gladiator midsize pickup model continued to spool up. Fiat Chrysler and its dealers delivered 16,132 Gladiators last quarter, more than doubling the number sold in the second quarter. Fiat Chrysler's total pickup deliveries (including the Gladiator as well as the company's Ram trucks) rose 25% year over year in the third quarter, reaching 177,767 units.

A red Ram pickup truck parked under a highway overpass.

Fiat Chrysler's Ram brand continues to gain market share rapidly. Image source: Fiat Chrysler.

A credible three-way race is developing

GM and Ford still sell far more trucks than Fiat Chrysler. Last quarter, they outpaced the latter by 47% and 38%, respectively. But Fiat Chrysler's Ram brand continues to gain market share, while the new Jeep Gladiator is quickly gaining traction in the midsize truck market.

At the same time, General Motors made it clear last quarter that it isn't conceding the leading position in the truck market to Ford. After a pair of ugly quarters, GM led the domestic industry in full-size truck sales and total truck sales in the third quarter.

The recent upheaval in the industry pecking order seems likely to continue. The ongoing strike that has crippled GM's North American production since mid-September will likely constrain the company's truck sales in the fourth quarter, giving Ford a golden opportunity to retake the top spot -- and enabling Fiat Chrysler to make further market share gains. However, looking into 2020 and beyond, it's anybody's guess how the market share battle between these three U.S. automakers will develop.