What's up with Carl Icahn's sudden car obsession?

Carl Icahn
Peter Foley | Bloomberg | Getty Images

Carl Icahn is experiencing a sudden car craze.

Last week, the activist investor plunged $100 million into ride-sharing start-up Lyft. And in Monday's letter to Apple CEO Tim Cook, Icahn used the word car or automobile 23 times in talking up the company's growth prospects and per-share value of $240.

Let's not forget that Icahn spent 2014 snapping up shares of Hertz and now owns 12 percent of the rental car company. Hertz was his fourth-biggest holding as of December, behind Apple, and Chesapeake Energy.

Read MoreIcahn hitches ride with Lyft

Finding a connection among the three investments requires a fair amount of guesswork. Hertz has been saddled with accounting and operational problems that Icahn has pledged to fix. Lyft, by contrast, is a fast-growing start-up, and Apple likely won't be deep in the auto game until 2020, according to recent reports.

But Icahn is clearly doing plenty of research on the market and where it's headed. In his letter to Apple, urging the company to increase its buyback program while continuing to invest in ambitious growth projects, Icahn showed his wealth of knowledge in the movement toward driverless cars.

Carl Icahn: Apple worth $240/share today
Carl Icahn: Apple worth $240/share today

"As autonomous driving would release drivers' attention from the activity of driving and navigating, and perhaps even increase the time people actually want to spend inside a car, both an automobile and the services provided therein become even more strategically compelling," Icahn wrote. "The car market is more than big enough to 'move the needle' significantly, even as the world's largest company."

Icahn owns almost $7 billion worth of Apple stock, so his $100 million investment in Lyft at a $2.5 billion valuation is hardly a rounding error.

Still, he's throwing his hat in the ring of perhaps the most high-profile battle going on in start-up land, where Lyft ranks a distant second to Uber. He's bullish enough to make the announcement public and put a quote in a press release.

Read MoreLyft CEO: Flexibility the new stability

"I believe that ridesharing is poised to become a fundamental component of our transportation infrastructure," Icahn said in the statement. "The company's revenue growth to date has been extremely compelling, and increasing urbanization over the next five to 10 years should enable the company to maintain that trajectory."

Did Icahn and Lyft discuss the investor's broad auto industry thesis during the negotiations and where Lyft fits in? CNBC put in calls to both parties and haven't yet heard back.