After years of promises, Tesla on Thursday unveiled a car that Elon Musk, the company’s chief executive, said will be able to drive itself without human supervision. Mr. Musk has said the vehicle will add trillions of dollars to the company’s stock market value and fuel its growth.

The “robotaxi,” which Mr. Musk also referred to as a “cybercab,” will cost less than $30,000 and be available before 2027, he said. He acknowledged, “I tend to be a little optimistic.”

Investors, who had high expectations for the presentation, were disappointed by that timeline and the lack of details about the company’s plans. Tesla’s share price was down about 8 percent on Friday afternoon.

The product, which does not have a steering wheel or pedals, is a prototype of a self-driving taxi that Mr. Musk has staked Tesla’s future on.

Mr. Musk promised that the vehicle, which appeared to be made of stainless steel, will be able to ferry passengers to any destination without human intervention, a feat that other companies have achieved in just a few places, like Phoenix and San Francisco.

“You could fall asleep and wake up at your destination,” he said.

The much-anticipated unveiling event at the Warner Bros. studio near Los Angeles began almost an hour late after one of the invitation-only guests suffered a medical emergency, according to Mr. Musk. Guests were offered rides around movie sets, but some were unimpressed.