You've heard about the guy who went from a dead-end job to incredible wealth in just five years because of Uber. This story is sort of the other way around.

If you're in Boston and you use UberX, keep an eye out for a driver named Paul behind the wheel of a Tesla S P85D (which retails for more than $100,000). That will be Paul English, the co-founder of Kayak, which was sold to the Priceline Group for what turned out to be about $2.1 billion in 2013.

He's only driving part time--having picked up 26 customers since he started on October 31--and it's all about research for his new stealth-mode startup, which will focus on the travel industry.

"We're rethinking how people book travel, from how they conceive travel, who they get advice from, where to go, how much work it takes to book it," English told me this morning, in an interview before we each headed to our day jobs. "I always said even when we were at Kayak that we sucked less than other websites. I was trying to make it simple, but even with Kayak there was some work involved."

His research goal with Uber? Mainly to learn what the rating experience feels like.

"I wanted to study what it felt like as a driver, knowing that at the end of every ride, you were rated on a scale of 1 to 5," English told me. "And the drivers rate [passengers] as well. ... [They're] trying to reward kindness. I think that's been really good."

English co-founded Kayak in 2004. The company went public in 2012 and was acquired in 2013, after which English raised $20 million to start an incubator in Boston called Blade. In July, when his noncompete agreement with Priceline expired, he started to focus on this single idea. (English is also the chairman and co-founder of a nonprofit called Summits Education that runs 42 schools for 10,000 students in Haiti.)

He's given five stars to 25 passengers, but only one star to a single passenger who was abusive and angry, he said. Among his most memorable customers so far:

An eighth-grade girl from China and her mother, who were looking at Boston-area high schools. ("She said, 'It's my life's dream to go to MIT,'" English said.)

A group of people he drove from a downtown nightclub to South Boston. ("They'd been drinking, and I can't replicate her Boston accent even though I have one, but she looked at the 17-inch screen and said, 'You must be a millionaire!'")

The only fare, out of 26, who figured out who he is. "When people ask me what I do [for a full-time job], I say I'm a programmer," said English. "I don't want to talk about me; I want to learn about them. But I picked up this couple about 25 years old, and one's showing an iPhone to the other, and then one of them goes: 'Are you Paul English?'"

Perhaps the most surprising thing he's found, English told me, is that driving people around is actually kind of fun. He said he appreciates the fact that it gives him a chance to talk with people he would likely never run into otherwise.