In Washington, President Xi Jinping of China is largely considered a mysterious autocrat who has long harbored a fatalistic view of his country’s relationship with the United States.

But for a brief moment on Wednesday, at a century-old gilded mansion in the hills of Northern California, President Biden treated Mr. Xi like just another car guy.

“It’s a beautiful vehicle!” Mr. Biden said as he walked the Chinese leader to his car, an armored, 18-foot Hongqi sedan, after a long day of diplomatic wrangling, according to a video published by the state-run China Global Television Network.

“Show the president,” Mr. Xi said through a translator, chuckling. Mr. Biden, whose love of muscle cars is so well known that it has been skewered by the satirical news site The Onion, peeked inside Mr. Xi’s car, then gestured to his own ride, a squat armored Cadillac built like a rolling bunker.