Tesla is one of the most popular stocks for retail investors. If Elon Musk is to be believed, they should consider buying up the company’s cars instead.

The CEO declared at Tesla’s investor day Monday that the future launch of an on-demand robot taxi fleet of self driving Teslas will give the cars a net present value of about $200,000. Not a bad deal for speculators, given that the cars start at about 20% of that sum. Mr. Musk also promised that owners will be able to earn passive income when they put their cars into commercial service. That compares pretty favorably with Tesla shares, which don’t pay a dividend and have lost 20% of their value this year.

Of course, Mr. Musk has to catch up on a few other promises first. He set a goal for a Tesla to drive itself across the U.S. within a year—back in 2016.

Write to Charley Grant at charles.grant@wsj.com