Tesla stock started the week with a bang, rising 5.5% on Monday. Investors' optimism wasn't easy to explain. The likeliest reason, according to Future Fund co-founder Gary Black, was robo-taxis. Investors are getting more excited about a potential introduction of Tesla's Cybercab into its fleet of self-driving cabs.

The robo-taxi business is a big deal for Tesla. Still, investors might be getting ahead of themselves.

Tesla launched a robo-taxi service in Austin, Texas, in June 2025 to much fanfare. Growth, however, has been slow. Tesla now operates in a few cities, including Miami and Dallas. Black estimates the fleet size is only about 100 vehicles, much smaller than Alphabet's Waymo. Its fleet is closer to 4,000 cars operating in more than a dozen U.S. cities.

Tesla didn't respond to a request for comment about fleet size.

The Monday jump was "Tesla bulls getting ahead of themselves," says Black.

That might be the case. Tesla stock was down 1.75% in premarket trading Tuesday at $361.5.

Despite Monday's gain, Tesla stock was only a few dollars higher than late June 2025, after the robo-taxi launch. Investors are still waiting for Tesla's robo-taxi business to scale meaningfully.

To be sure, robo-taxis are coming. Others, including Amazon.com's Zoox, are offering rides. Waymo is even running ads during the U.S. Open tennis broadcasts.

Who will win the most robo-taxi market share is still hard to say. And how profitable robo-taxis will be amid rising competition is another question investors have to answer. It's an important question. Morgan Stanley, for instance, values Tesla's autonomous driving technologies at roughly $1 trillion, about six times the value ascribed to the car business.