By Nora Eckert, Kalea Hall, David Shepardson

DETROIT, March 5 (Reuters) - U.S. President Donald Trump's 25% tariffs on Canada and Mexico have sent the U.S. auto industry scrambling to plan for the massive tax on some of America's best-selling vehicles, including full-sized pickup trucks, while pinning their hopes on a potential deal in Washington.

Hours after the tariffs went into effect, the White House threw the industry a lifeline, saying many North American-built vehicles would be exempt if they already followed complex rules of the 2020 U.S.-Mexico-Canada Agreement's rules of origin, enacted during Trump's first term.

"We are going to give a one-month exemption on any autos coming through USMCA... so they are not at a disadvantage," White House press secretary Karoline Leavitt told reporters Wednesday. "Reciprocal tariffs will still go into effect on April 2."

Trump raised the idea of a 30-day pause on USMCA-compliant vehicles in return for expanding production in the U.S. during a call on Tuesday with GM CEO Mary Barra, Ford CEO Jim Farley, Ford executive chair Bill Ford Jr. and Stellantis chairman John Elkann, Reuters reported earlier.

Automakers have expressed support for boosting U.S. investment but want certainty over tariff policies as well as vehicle emissions rules before making dramatic changes, two auto sources said.

Ford, GM and Stellantis declined to comment on the meeting.

Such a deal could be an especially welcome development for pickup-truck makers - and for their leading customers, who lean heavily towards Trump's rural base of Republican voters.

About a third of U.S. pickups sold by American and foreign brands are manufactured in Mexico and Canada, according to research from Global Data.

The quintessentially American product is the backbone of the U.S. car industry, providing large portions of the sales and profits for General Motors, Ford and Stellantis, owner of the Jeep and Ram truck brands. Automakers, including U.S. and foreign brands, sold nearly 3 million U.S. pickups last year, about 20% of overall national sales.

And pickup drivers are about twice as likely to say they are Republicans than Democrats, according to an August survey by Edmunds, an industry information provider.

The tariff pause gives the industry additional breathing room to hold consumer prices steady because of existing inventory on dealer lots. Ohio dealer Rhett Ricart, who sells GM and Ford vehicles among other brands, held out hope for a quick deal to avert a crisis.

"I think it won't take a month for them to figure out how to handle this thing," he said, speaking before Wednesday's announcement. "I'll be more concerned ... 30 days from now."

'NO CHOICE' BUT TO PASS ON COST

Trump's tariff threats has automakers and suppliers gaming out how they might avoid or absorb such taxes — and how much they might have to raise consumer prices. The answers to such vexing questions could vary widely by automaker, depending on their and their suppliers’ exposure to Canada and Mexico manufacturing.

Analysts at Wolfe Research projected the tariffs would add about $3,000 on average to the cost of a vehicle, and around $7,000 on models imported from Canada or Mexico. Full-size pickups have an average transaction price of about $65,000, according to January data from Cox Automotive.

"Once the manufacturer starts passing on that cost to us, we're going to have no choice but to pass it on" to consumers, said Jeff Tamaroff, Chairman Of Tamaroff Auto Group, which owns Honda and Nissan dealerships in Michigan.

Those extra costs would come in addition to already-soaring vehicle prices, which started to rise sharply during the coronavirus pandemic and haven’t eased much since. The average vehicle sales price hit $48,641 in January, according to Cox Automotive data.

Among Detroit brands, GM's Chevrolet and GMC pickups, along with Stellantis's Ram, are more exposed to Trump's taxes than Ford because both build large numbers of pickups in Mexico.

Ford builds its F-series pickups in the United States - but also makes some truck engines in Canada, underscoring the web of economic interdependence among the three North America trading partners.

Almost no American vehicle is made from solely American parts, industry research shows.

Barclays bank analysts estimate that Mexico provides up to 40% of the parts in U.S. vehicles and Canada more than 20%. Suppliers say they will have to cover some of the tariff costs and will likely see an additional hit if consumer demand weakens from rising vehicle prices.

Automakers and suppliers also worry about the effects of tariffs on vehicle components that bounce across borders before reaching their final destination. Companies worry that such parts could be taxed with every border crossing, although Trump has not clarified his policy in such cases.

Take truck transmissions made by German supplier ZF Friedrichshafen. The transmissions, which are used in the Ram and other vehicles, include parts that cross borders several times before they reach their final destination.

'HOWEVER BAD IT LOOKS, IT'S WORSE'

The journey begins at a factory in Mexico, which produces torque converters, a donut-shaped part that transfers power from the engine to the transmission. The torque converters are then shipped to a ZF facility in South Carolina that assembles the transmission. The finished transmission, including the torque converter, is then sent back to Mexico for installation into a Ram pickup - which then crosses the border again to reach a U.S. dealer.

About 30 other car and truck components follow a similar zigzagging path across the U.S.-Mexico border, ZF said.

"A tariff over Mexico, in this particular case, or over Canada, is going to mean hundreds of millions of dollars as an impact" on the industry overall, said ZF North America President Ramiro Gutierrez.

Without major production shifts, the ripple effect across thousands of individual auto parts could potentially reach $40 billion by the end of 2025, according to Bernstein analysts.

“However bad it looks, it's worse,” said Pat D’Eramo, the chief executive of Martinrea, a Canadian company that makes brake lines and other products. The company has manufacturing operations in all three North American countries with some products crossing borders multiple times.

The American auto industry, which for decades has enjoyed free trade across the U.S., Canada and Mexico, is now contemplating how to adjust supply chains if the trade war continues, a potentially expensive prospect.

U.S. automakers and their suppliers have invested billions to expand their U.S. footprint and avoid tariffs since the Trump administration enacted USMCA in 2020 to replace the 1994 North American Free Trade Agreement.

Now, some industry executives say they're being punished for complying with Trump’s signature trade deal.

"This is a bonanza for our import competitors," Ford CEO Jim Farley told analysts last month of Trump’s new tariff threats, pointing out that some rivals import from Asian countries with few duties.

(Reporting by Nora Eckert and Kalea Hall in Detroit, David Shepardson in Washington, Eric Cox in Detroit; Editing by David Gaffen, Brian Thevenot and Suzanne Goldenberg)

((Nora.Eckert@thomsonreuters.com;))