(Reuters) - President Donald Trump on Wednesday followed through on weeks of threats for new tariffs on imported cars, saying a 25% import tax on vehicles not built in the U.S. would kick in next week.

Trump will impose a 25% tariff - on top of previous duties - on imports of finished vehicles starting at 12:01 a.m. EDT (0401 GMT) on April 3. The base U.S. tariff rate for automotive imports is 2.5%.

The plan, set to upend the auto trade and supply chains, fanned uncertainty among customers and investors, threats of retaliation and pummeled global auto stocks from Asia to America.

Here's what is known so far.

LEAST TO MOST HIT

Half of the cars sold in the U.S. last year were imported, according to research firm GlobalData. General Motors ( GM ) imports 46% of its car sales, and Ford a relatively lower 21%.

Both GM and Ford source many parts from outside the U.S., with a significant share coming from Mexico. These companies may be under pressure until further clarity emerges regarding finished vehicles and auto parts on April 2.

Tesla would be less impacted, since all its production and assembly are done domestically. Automakers could ramp up efforts to localize production to offset tariff costs, benefiting domestic suppliers in the long term.

The shift could disrupt global supply chains as companies restructure sourcing strategies and manufacturing operations.

The duties will be applied to cars and trucks built in countries that have free-trade agreements with the U.S., including Canada, Mexico and South Korea. They will be hard-hit, as will be Japan and European Union auto producers Germany and Italy, along with Britain.

DELAY IN 25% PARTS DUTIES

The 25% tariffs will also apply to major automotive parts imports, identified in Trump's proclamation as "engines and engine parts, transmissions and powertrain parts, and electrical components." But parts duties may start up to a month later with a date to be set in a forthcoming Federal Register notice, but not later than May 3.

The notice will also contain the specific tariff codes for components subject to the duties, which were not revealed in Trump's proclamation.

PARTIAL USMCA EXEMPTION

The plan provides a partial exemption from tariffs for vehicles and parts that comply with the USMCA's rules of origin, but only for the value of their U.S.-produced content. So a truck built in Mexico with 45% U.S. content would still face a 25% tariff on 55% of its value.

The same concept would apply to auto parts that are compliant with USMCA rules of origin - the non-U.S. content gets taxed. 

But determining these content levels will be complicated. Until the Commerce Department and U.S. Customs and Border Protection agency determine a process to apply tariffs to their non-U.S. content, USMCA-compliant auto parts will stay duty-free. There was no deadline specified for the process.

WHAT ABOUT AUTO RETAILERS AND SUPPLIERS?

Auto retailers are staring at higher costs, as imported vehicles and parts become more expensive. This could lead to steeper sticker prices, weakening demand and slowing sales.

Suppliers reliant on international markets may find it difficult to absorb the tariffs or pass the costs onto automakers, squeezing profit margins.

Analysts at J.P. Morgan noted that franchise dealers' parts and services business might benefit, as higher prices could prompt customers to hold on to their existing cars longer, increasing demand for repairs and maintenance.

LEGAL RATIONALE

Trump's new tariffs are based on a 2019 national security investigation into auto imports conducted during his first presidential term under Section 232 of the Trade Expansion Act of 1962. Trump previously used this Cold War-era trade law to impose 25% tariffs on steel and aluminum imports in 2018.

The Commerce Department found that the growing market share of imported cars was negatively impacting U.S. national security by eroding the U.S. industrial base and the ability of domestic automakers to develop advanced technologies for military use.

Trump at the time chose not to impose tariffs, opting instead for negotiations with trading partners to remedy these concerns.

But on Wednesday he concluded that these talks had failed, the security threat from imports had worsened, and revisions to USMCA and KORUS had not improved the U.S. position in automotive trade.

(Reporting by David Lawder, Andrea Shalal, Nathan Gomes and Anshuman Tripathy; Editing by Lincoln Feast and Sriraj Kalluvila)