Once or twice a week, towards the end of the day, Jerry Seinfeld leaves the Manhattan office where he spends his afternoons writing, but he doesn't head home to his family. Instead, he shows up unannounced at some minor comedy club in New York or New Jersey, and inserts himself into that night's lineup. Seinfeld reportedly has a private jet, owns more than 40 vintage Porsches and makes at least $32m (£19.5m) a year, in large part from syndication revenue, adding to a net worth estimated at $800m (£487m) in 2010. Having spent a decade making a celebrated "show about nothing", he could easily afford to just do nothing now. But he prefers – or feels compelled – to keep honing his act, trying a new line here, shaving a word off an old one there, analysing the audience's laughter: a scientist of comedy, painstakingly calibrating his equipment. By the time you hear a Seinfeld "bit" at one of his £70-a-ticket O2 Arena gigs, or on a TV talkshow, it will have undergone months or years of testing, and there won't be a syllable wasted. As in: "Why does moisture ruin leather? Aren't cows outside a lot of the time?" Or: "A two-year-old is like having a blender, but you don't have a top for it." Or: "People are never really sure if they have milk."

Because his material's never edgy or obscene, because his delivery is so laidback, and because he's so stupendously wealthy, it's easy to dismiss the Seinfeld of 2014 as altogether too slick, smug and mainstream. But in his best lines, buffed to a perfect shine, extreme professionalism crosses over into a kind of absurdist Zen – and that takes work. "To a guy like me, a laugh is full of information," Seinfeld says, on a bright winter morning in a photo studio three floors above Broadway. He has just posed for pictures in one of his trademark designer suits, but now he's back in dark jeans, a sweatshirt and blue-and-green sneakers, his tall frame slouched in a tatty armchair. "The timbre of it, the shape of it, the length of it – there's so much information in a laugh. A lot of times, you could play me just the laughs from my set and I could tell you, from the laugh, what the joke was. Because they match."

This April, despite being lodged in the collective consciousness as permanently fortyish, Seinfeld turns 60, which is the kind of age he used to tell jokes about. ("My folks are moving to Florida. They didn't want to move to Florida, but they're in their 60s, and that's the law.") He has become a grand old man of standup, a national treasure; don't audiences laugh simply because he's Seinfeld, especially if they hadn't been expecting him? "Maybe for a couple of minutes. But I've said it many times: nobody laughs at a reputation. They get excited in the beginning, but they can't lie to me. They cannot lie to me. I'm going to find out."

For years after the finale of Seinfeld in 1998 – an event only beaten, in sitcom viewership history, by the finales of M*A*S*H and Cheers – people speculated about his next major project. There was the 2002 documentary Comedian; and there was 2007's Bee Movie, a modest box-office success, which Seinfeld co-wrote and produced, and in which he voiced the lead role of a bee outraged to learn that humans are stealing honey. But even as he promoted it, he was ruling out the possibility of a Hollywood producing career. ("Oh my God, I'd kill myself," he told one interviewer. "Give me a gun.") Then came The Marriage Ref, a shortlived combination of gameshow and reality series in which celebrities cast judgment on real-life couples' marital disputes, panned in both its American and British incarnations.

These days, an alternative possibility is beginning to suggest itself: what if the post-Seinfeld Seinfeld just isn't a "major projects" kind of guy? His latest creation, a web series entitled Comedians in Cars Getting Coffee, has frequent moments of brilliance, but it's not even a show about nothing; it's barely a show at all. In each of the episodes, which vary in length, Seinfeld collects a fellow comedian in a different vintage car (Chris Rock, Larry David, Mel Brooks and Ricky Gervais are among the participants); they then drive to a diner or cafe, drink coffee and talk. "The real action of the show," the New York Times's reviewer explained, "consists of this seemingly involuntary snorting, cackling laughter of middle-aged men so amused by each other's observations … that they rock in their seats and double over in helpless paroxysms." Actually, there are some women, too: the third season, which starts this month, includes Tina Fey. But apart from that, the analysis isn't unfair. What's strange is how well the idea works.

"My goal was to make it the effortless talkshow, where you don't have to show up, you don't have to think about what you're wearing, there's no makeup, there's no prep – there's nothing. It's literally getting in a car." The effort comes in the editing, but that's another opportunity to smooth and hone obsessively, "so I kind of enjoy it". The format doesn't allow for audience-tested Seinfeld one-liners, so much of the pleasure comes from the other comedians' contributions. One of the best episodes largely involves Mel Brooks eating pastrami in Carl Reiner's living room and retelling old jokes: "Guy gets hit by a car, little Jewish man, his friend says: 'Get a pillow! Do something! Put it under his head! All right. Are you comfortable?' And the guy says: 'I make a living …'"

A competing theory for Seinfeld's low profile since 1998 is that his comedy belongs squarely to the 90s – an era of economic plenty, before 9/11, before widespread anxiety about climate change, when the bottomless self-absorption of Jerry, Elaine, George and Kramer felt excusable. Rewatching the show today is a curious experience. The haircuts are terrible, obviously. But the much-hyped focus on "nothing" – on overblown conflicts with doormen, restaurateurs and so on – feels familiar: it's central to many of the shows that count Seinfeld as a major influence, from Arrested Development to The Office to Curb Your Enthusiasm. (The latter's success fuelled yet another theory about Seinfeld's post-90s career: that Larry David had been the genius behind the sitcom all along.) What stands out, in those old Seinfelds, is the weird callousness: a total lack of concern with anyone other than the central foursome, unmatched even by Larry David's character in Curb, or David Brent, or the South Park kids. When George's fiancee dies, poisoned by the glue in the cheap wedding invitations he'd insisted on buying, his pure relief is certainly funny, and in keeping with the famous motto of the show's writers: "No hugging, no learning." But it's also more pathologically egocentric than anything you'd encounter, in a comedic context, on TV today.

The real-life Seinfeld has little time for this kind of analysis, professing zero interest in capturing zeitgeists, or in the postmodern themes that academics love identifying in his work. (He even claims, semi-convincingly, not to know what people meant when they called the sitcom "meta": "I've looked it up a number of times. Could you define it for me?") His preferred view of himself isn't the cliched one of the tortured comedian battling inner demons, but of a baseball player or a sprinter, obsessively cultivating a single skill, fighting not to lose his edge. "I think of myself more as a sportsman than I do an artist," he says, which explains his bafflement when he's asked, as he often is, about whether he'd like to take acting roles in films. "It's hilarious to me that anyone would think I would have the slightest interest in it. Baseball players don't think: 'I gotta get into soccer!' They think: 'I gotta do what I gotta do to try and hit that ball today. That's my life.' As a comedian, I found this thing, this profession, that suits my mind and life force. To drop it to do something else? I just don't get that."

Keeping his act sex- and swear-free, the way he sees it, is part of this athletic challenge, since it denies him the easiest laughs: "A person who can defend themselves with a gun is just not very interesting. But a person who defends themselves through aikido or tai chi? Very interesting." Likewise his focus on minutiae. "It's so much easier when you're talking about something that really is important. You've already got a better foundation than someone who's bringing up something that does not need to be discussed." Such as? "I do a lot of material about the chair. I find the chair very funny. That excites me. No one's really interested in that – but I'm going to get you interested! That, to me, is just a fun game to play. And it's the entire basis of my career."

The cast of Seinfeld. Photograph: Nbcuphotobank/Rex Features

One episode of Comedians In Cars Getting Coffee does attempt to get more serious: it features Seinfeld's former co-star Michael Richards, and concerns the shockingly racist tirade , directed at black hecklers, that ended Richards's standup career in 2006. "It was a very bad miscalculation," Seinfeld says today. But a miscalculation, he insists, is what it was; it's a misunderstanding of standup to conclude that it showed Richards to be a racist himself. "Lenny Bruce already did that bit! He already did it! It's a knife-throwing act, and unfortunately Michael missed." On camera, Richards seems genuinely remorseful, and resigned to never working again.

In a recent New York Times profile of Seinfeld, another guest on the series, Sarah Silverman, called him "the least neurotic Jew on earth". But his early life was highly typical for Jewish New York. The son of Austrian and Syrian immigrants, he was raised in the Long Island town of Massapequa, which he likes to say is "an old Indian name, meaning 'by the mall'". The family kept strictly kosher and attended synagogue; the teenage Jerome spent time on an Israeli kibbutz. He seems never to have considered any career but comedy: as a college student in Queens, he managed to persuade his tutors to let him study standups – and perform himself – for course credit. (Jackie Mason, in the audience one night, delivered praise that kept him going for years: "It makes me sick, you're going to be such a big hit.") In 1981, he got his big break: the first of many spots on Dick Cavett's talk show. Seven years later, over coffee in a New York diner (of course) with Larry David, the sitcom, originally entitled The Seinfeld Chronicles, was born.

All the way through, from first standup shows to stardom, he forced himself to work by marking a cross on a calendar for every day he wrote material; soon enough, he had a long chain of crosses, and kept going partly because he didn't want to break the chain. Since he revealed this trick to a would-be comedian years ago, "Seinfeld's Productivity Secret" has achieved cult status online: there are at least three apps and one website dedicated to helping people emulate it. This amuses its inventor no end. "It's so dumb it doesn't even seem to be worth talking about," he says. "If you're a runner and you want to be a better runner, you say, well, I'll run every day and mark an X on the calendar every day I run. I can't believe this was useful information to anybody!" He spreads his palms, a gesture conveying the sheer obviousness of the insight. "Really? There are people who think, 'I'll just sit around and do absolutely nothing, and somehow the work will get done'?"

Jerry and Jessica Seinfeld in Central Park, New York. Photograph: Jamie Mccarthy/Getty Images for Baby Buggy

Earlier in his career, it was Seinfeld's life outside work that consumed the media most: first when, in his late 30s, he began dating the 17-year-old Shoshanna Lonstein, now a fashion designer; and again at 45, when he began a relationship with a PR executive, Jessica Sklar, made public mere weeks after she'd returned from honeymoon with her husband, the scion of a family of Broadway theatre owners. But 15 years later, he touts the conventionality of his domestic arrangements: he and Sklar, now Jessica Seinfeld, have three children aged between eight and 13, and he is openly judgmental of celebrities incapable of such stability. "Comedians are known for having long marriages," he says. Why? "I have to apologise for the self-serving answer I'm going to give you, but: we're smart. If you're smart, you stay married if you can. Marriage is hard for everyone – that's a basic fact – but it's a better life if you can do it. Very nice. Very relaxing. Very enjoyable." One part of his standup routine stresses the importance of realistic expectations. "Here's the secret I'm going to tell you about relationships," he tells his audience. "They don't work … Do you know what works? Potato chips work. Fire extinguishers work. Relationships? They don't work."

Seinfeld has a theory about marriage and driving, which may explain his choice of post-sitcom projects. "They're the window into every aspect of human life," he claims. "There's just two things I'd need to find out everything I want to know about everyone: one, let me see them drive; two, let me hear them talk about marriage, whether they're married or not. That's going to tell me exactly your relationship to the world. I wouldn't need to know anything else." If he had the time, inclination or energy, he says – "and I have none of those three" – he'd make The Marriage Ref again, "just because I do find marriage to be the most interesting subject there is." (If there is a next time, I recommend not including Alec Baldwin on a panel of experts on marital harmony.)

The subject also offers him a guaranteed point of connection to audiences, counteracting the ironic fate of the extremely successful observational comic – which is that once you're travelling in private jets or a fleet of vintage Porsches, the life you're observing isn't one your audiences know. ("I may be dumb," he says, when I ask if he ever talks about his car collection in his act, "but I'm not that dumb.") Leaving LA also helped with that problem. "That's the nice thing about living in New York: just being on the street does a good job of breaking [the celebrity bubble]," he says. "LA? Not so much. You leave your gorgeous house, get in your gorgeous car, and go to some gorgeous place where you're feted. It's tougher. That was a conscious choice of mine, when I lived in LA: I thought, if I want to stay funny, I need to get out of here."

Jerry Seinfeld on stage in New York in 2013. Photograph: Bryan Bedder/Getty Images for New York Comedy

To this day, Seinfeld still marks crosses on a calendar, keeping regular hours (albeit relaxed ones: most days, he says, he'll meet a friend for a two-hour breakfast) and spending 20 minutes a day doing Transcendental Meditation, which is the only topic to jolt him from his default nonchalance into real enthusiasm: "I could do the whole interview about TM, to be honest, but we'd just lose everybody. I'll describe it very simply: it's like you have a phone, and somebody gives you a charger for it. And so now you can recover from this exhausting experience of being a human, twice a day. It's deep rest. Now that's something that can help people. As opposed to this idiotic calendar thing."

Seinfeld talks about his comic routines as if they're discovered rather than created: observations that are out there, camouflaged against the patterns of everyday life, waiting for him to detect them. One example: the other day, his two sons were arguing, because one of them had farted. "They were accusing each other – 'he who smelt it dealt it!' – and I just thought, Jesus, these guys need some new material. That's the same thing I was saying when I was five. Fifty years ago! Kids! I can't believe they're still doing the same material!"

"Kids need new material": that was an idea with legs. With a bit of work, there could be a few minutes of an act there. Some things just have potential while others don't; he doesn't claim to be able to explain why. Chairs are inherently amusing. Salt-shakers, Seinfeld reckons, are not. Reflecting upon the comic possibilities of various other everyday objects, a thoughtful expression passes across his face. "I find the fork very funny," he says.

The third season of Comedians in Cars Getting Coffee has begun at comediansincarsgettingcoffee.com