HOFU, Japan — Mazda, the longtime also-ran of Japanese automakers, says it came up with innovations in nearly every step of auto manufacturing for a super-efficient assembly line that rolls off vehicles at a stunning rate of one every 54 seconds.

The revamped Hofu plant in Yamaguchi Prefecture, southwestern Japan, shown to reporters Tuesday, underlines how Mazda Motor Corp. has defied skeptics who predicted the automaker’s demise after Ford Motor Co. ended a long partnership.

Contrary to expectations, Mazda was not bought by a Chinese competitor. Nor did it collapse under the burden of a soaring yen that made Japanese cars more expensive abroad.

Mazda is still riding on its reputation for producing cool gas-sipping models such as the Miata roadster without a single gas-electric hybrid in its lineup. The Hofu plant can barely keep up with demand. Its pace betters that of Toyota Motor Corp., the world’s top automaker, which can roll out a vehicle at paces varying from 57 seconds to 115 seconds.

The key to what Mazda calls its innovation in “monozukuri,” or “making things,” apparent at the Hofu plant, was using a common platform, the main structure on which a car is built, and common parts. Platform-sharing is a standard profit-boosting device in the auto industry, but is even more crucial for a smaller player such as Mazda, allowing it to create several distinct models from what in basic ways is the same car. After its partnership with Ford ended three years ago, Mazda needed a new approach.

Mazda officials said it will include all the innovations at its new plant in Mexico, set to go into production next year.

The Hofu plant, first opened in 1981, rolled its 10 millionth car, a Mazda6 sedan, off its line Tuesday.

“We see this as one step toward further growth,” President Masamichi Kogai said at a roll-off celebration, where workers set off party crackers and shouted, “Go for it.”

The Hofu plant produced 350,000 vehicles last year, down from its peak at above 500,000 in 2007, but it’s recovering this year to about 400,000 vehicles.