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, eBay 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.