U.S. defenders of the Trans-Pacific Partnership point out that Detroit's automakers will still be shielded by a rollback of U.S. tariffs so gradual that it spans decades.
But that misses a big point.
First, Japan-brand sales in the U.S. have hardly been hobbled by the 2.5 percent tariff on imported passenger cars and 25 percent "chicken tax" on trucks. On the contrary, thanks to localized production, their business has been booming. Eliminating tariffs, no matter how gradually, simply gives Japan brands another welcome tailwind.
The battle really isn't about access to the U.S. market. A bigger prize is the booming economies of other countries in the pact: those in Latin America, such as Chile and Peru, and Southeast Asia, such as Vietnam and Malaysia.
That's where Detroit and Japan are jousting for the next wave of sales growth. Unfortunately for Detroit, Japan already vastly out-exports the U.S. to those countries.
The reported agreement to set required local content at 45 percent should give the Japanese another nudge. Their factories in Japan can ramp up imports of cheap parts from neighboring low-cost countries such as China and still qualify for free trade.
That is a geographical advantage factories in the U.S. can't match.
Finally, many backers argue that Japanese automakers are unlikely to benefit much from free trade with the United States because they have already off-shored so much production from Japan to the United States. Indeed, annual output of Japan brands in the U.S. skyrocketed to 3.81 million vehicles in 2014, from just 296,569 in 1985.
But free trade cuts both ways. Just as it opens export markets for Detroit brands manufacturing in the U.S., it opens new export markets for Honda plants in Ohio, Toyota plants in Kentucky and Nissan plants in Tennessee.
Even without the trade deal, Japan-brand exports from the U.S. surged 129 percent to 391,336 vehicles in 2013 from 2009, and climbed to 472,000 in 2014. That is more than double the 233,145 vehicles exported by General Motors last year.
To be sure, any boost in Japan-brand exports from the United States would help American workers. But would it really help homegrown auto manufacturers and their dependents?
If anything, it gives Japan a nice global hedge, with export engines both in its home market and in North America. When the yen is too expensive, it can now more easily ramp up exports from the United States, and when it's weak, fire up factories in Japan.
Trade pacts as expansive as TPP, which covers everything from computer chips to coconuts, inevitably involve trade-offs. As a whole, the United States economy may benefit.
But zoom in closer, and it appears the Japan auto brands scored an advantage on their Detroit rivals.