$Tesla(TSLA.US)$ Chief Executive Elon Musk said the true test for rival electric-vehicle maker $Rivian Automotive(RIVN.US)$ will be high production and break-even cash flow.

"There have been hundreds of automotive startups, both electric & combustion, but Tesla is [the] only American car maker to reach high volume production & positive cash flow in past 100 years.," Mr. Musk said in a tweet Thursday.

"I hope they're able to achieve high production and breakeven cash flow. That is the true test," Mr. Musk added, referring to Rivian.

A representative for Rivian wasn't immediately available for comment.

Mr. Musk's tweet on Thursday comes after Rivian's blockbuster initial public offering earlier this week. In its first two days of trading, the electric-car maker has already cinched a market value of more than $100 billion, roughly a 10th the size of Tesla.

Rivian's debut this week in many ways defies typical market fundamentals: The startup is barely generating revenue and is still losing money while Wall Street is already valuing it more than Ford Motor Co. and General Motors Co.

The rally shows the optimism around the auto market's shift to vehicles that run on electricity rather than gasoline -- a technology that has created an open field in the race to dominate the future automobile.

Mr. Musk has previously commented on Rivian, saying last month that "prototypes are trivial compared to scaling production and supply chain."