There are many reasons for the steady success of the three major American automakers in recent years, but none are bigger than the surging sales of full-size pickup trucks.
General Motors, Ford Motor and Fiat Chrysler dominate the segment in the United States market, and they rely on pickups for a sizable portion of their earnings in North America as a whole.
But President Trump’s proposed border tax on imported vehicles could throw a wrench in the profit machine, particularly for G.M. and Fiat Chrysler, which build a large percentage of their pickups in Mexico.
At the same time, Ford — which makes all of its pickups in American factories — would most likely benefit at the expense of its rivals.
The Trump administration has pledged to levy tariffs of up to 35 percent on vehicles imported from Mexico and possibly elsewhere, primarily to prod automakers to increase production and jobs in the United States.
In response, all three of the Detroit companies have announced plans for new investments in their American operations. Ford went a step further by canceling plans to build a $1.6 billion plant in Mexico that had been criticized repeatedly by Mr. Trump.
Yet the possibility of tariffs on trucks overshadows the give-and-take so far between a new president determined to expand the American economy and automakers that count on pickups as a huge source of income.
“There is an awful lot at stake with a border tax on trucks,” said Michelle Krebs, an analyst with Autotrader.com, a car-buying site. “These are the cash cows for Detroit.”
Pickup sales have been an important component of the consistent growth of the American auto market, which last year set a record with nearly 17.5 million vehicles sold.
The combination of low gas prices and a need by businesses to replace older trucks has stimulated demand, especially for full-size pickups, more than 90 percent of which are made by the three Detroit companies.
In January, the three top-selling vehicles in the nation were pickups: the Ford F-150, G.M.’s Chevrolet Silverado, and Fiat Chrysler’s Ram model. The companies earn an estimated $8,000 to $10,000 in profit on each truck sold — compared with $3,000 on a passenger car — so Detroit’s financial fortunes inevitably rise and fall on the success of their truck products.
Yet for G.M. and Fiat Chrysler, much of that success is dependent on truck production in Mexico, where labor costs are significantly lower than in unionized factories in the United States.
Industry analysts estimate that one-third of G.M.’s annual pickup production comes from its sprawling assembly plant in Silao, Mexico. And while Fiat Chrysler is expanding its American output of trucks, it still relies on its factory in Saltillo, Mexico, for 30 to 40 percent of its pickups, the analysts say.
Ford, by contrast, makes its pickups at three plants in the United States.
Automakers do not reveal profit margins on particular vehicles. But companies pay workers less than $10 an hour in Mexico, versus the top union wage of $29 in American plants. That differential makes Mexican trucks more profitable for G.M. and Fiat Chrysler than those produced in the United States.