In a city known for big swings and seven-figure seed rounds, Reed Kirk Rahlmann’s idea was almost painfully small.

He started his own tour company driving a Mini Cooper, a car that — concierges, competitors and strangers love to point out — will never carry more than a trio of paying customers.

“Even recently I was talking to this guy in a bar about what I do,” Rahlmann says. “And he goes, ‘You got to get a bigger car. … What if there are four people? How are you going to scale up? ’ I said, ‘Oh, I don’t think I ever want to do that.’ ”

Rahlmann’s business, Small Car, Big Time Tours, gives the smallest human-guided vehicle tours in San Francisco (even the three-wheeled Lucky Tuk-Tuk tour holds six passengers). His history-filled custom journeys can range from Monterey to Muir Woods, but many focus squarely on San Francisco. And he may be the city’s best cheerleader of the doom loop era, cleansing visitors of their “San Francisco is a hellscape” misconceptions, no more than three at a time.

Meeting on the easternmost end of California Street on a crisp Monday where bright blue skies split the skyscrapers, Rahlmann’s parked car looks almost comically small, surrounded by a delivery truck, large sedan and two cable cars.

But the nimble advantages of a vehicle that’s just 151 inches long are apparent as soon as he zips away from the curb. 

With the top down, it feels like a magic carpet ride through the city, hosted by the ultimate insider. In one 10-minute cruise from Grace Cathedral to North Beach, Rahlmann covers the history of Charles Crocker’s spite fence and the origins of Chinatown, names his favorite little-known public bathrooms (near Musee Mecanique at Fisherman’s Wharf and behind the California Palace of the Legion of Honor) and declares the free admission to the cable car barn and powerhouse one of the best deals in the city.

“That’s how the cable cars get around!” Rahlmann says, as if he’s seeing it for the first time, instead of the 500th.

He follows this, like many anecdotes on our 1½-hour tour, with a burst of non-sequitur positivity. “This is one of the benefits of being in the Mini. You look up and you can see the city. Man, it’s stunning today!”

The Lafayette native fell in love with San Francisco during his own family visits.

“I just thought, ‘Oh my God, this place is amazing.’ It always felt like my city,” Rahlmann says. “I moved here in college and stayed, and just became one of those hopelessly romantic San Franciscans.”

Rahlmann attended San Francisco State University and from there took the most circuitous (and most San Francisco) route imaginable to his current job.

He was an improv comedian in the early 1980s; performed at Pier 39, walking on broken glass and driving a 4-inch nail into his nasal cavity; and worked more recently as a mentalist-for-hire under the name Sebastian Boswell III.

He’s also worked as a writer — for Playboy magazine, New West and the Chronicle — and acted in local television and movies, including “Howard the Duck” (1986) and “Leonard Part 6” (1987).

“Had I been in ‘Ishtar,’ ” Rahlmann quips, “I would have scored a trifecta for appearing in the biggest bombs of the late 1980s.”

“They asked, ‘Tell us three things you think we don’t know about San Francisco,’ ” Rahlmann said. “And I knew this was my wheelhouse, because I spent my entire adult life studying San Francisco, exploring San Francisco.”

When Mr. Toad’s shut down in 2014, Rahlmann started his own tour business. He wanted a fun car — a convertible for fresh air, a flexible tour and small groups so he could add a personal touch. The Mini Cooper seemed like a perfect fit.

“It’s a great car to see the city,” Rahlmann says. “I want people to leave with more than just a postcard. I want them to have a real sense of San Francisco — what makes it a unique and special place.”

Rahlmann goes all in on presentation. Standing outside the car in a tie and vest that match the exterior’s “British Racing Green” color, he immediately hooks up a microphone, using the Mini’s speaker system to amplify his voice, even though his most distant passenger is about 3 feet away.

But the experience is choose-your-own-adventure throughout. One married couple wanted a tour of San Francisco built around the places they met, lived and enjoyed together, so they could show their 13-year-old son. Another pair wanted to drive up the coast and photograph nature.

And some passengers, no judgment, just want to see the sights in the snow globe they purchased near Fisherman’s Wharf.

“A lot of people will say, ‘I want to go down Lombard Street, but I know it’s touristy,’ ” Rahlmann said. “And I tell them, ‘Lombard Street is fantastic. You do not have to be shy about that.’ You know, most of the things that are touristy are really freaking interesting.”

With each group getting a personalized tour, often with many phone calls and emails exchanged in advance (at no extra cost), Rahlmann says his preparation can exceed the time on the road.

“And that’s fine. That’s part of it,” he says. “I want them to have the best experience possible.”

Driving on steep hills in his personal auto is another challenge — the Mini has manual transmission — to the point where Rahlmann can’t immediately answer accurately how many times he’s replaced the clutch on the 9-year-old vehicle. (He thinks it’s twice, but maybe three times?)

But Rahlmann can park in tight spaces and make a quick three-point turn on a narrow street or wide alley (try doing that in a Gray Line bus), feeding the “we can go anywhere” vibes of his tour guide philosophy.

New York resident Kenny Chen, a recent client, booked Small Car, Big Time Tours with Ricardo Senno; the childhood friends have gathered each year for the last decade to sample food in different U.S. cities.

Chen immediately knew he was not on a typical tour when they started getting calls and texts from their guide, to fine-tune a tour that included several major landmarks, Gold Rush lore and some food tips for their stay.

“It was so detailed, with so much history,” said Chen, who rated his trip with Rahlmann as the best he’s taken. “We’ve done a lot of tours and nobody pulls out an iPad and starts showing (photos) and telling you how things were 100 years ago.”

Rahlmann says he notices tiny shifts in the city through his work, such as how GPS mapping technology on phones has changed traffic patterns, clogging what were once his shortcut streets. Today, he gets more requests to see the “Mrs. Doubtfire” house at 2640 Steiner than some major landmarks. While tourists still dominate his bookings, more locals seem interested in exploring their backyard and learning the history.

Other changes are less subtle, including a couple of years ago, when out-of-town tourists began telling him, by way of introduction, “Oh, San Francisco. They told me it’s a hellscape.”

Inevitably, he says, the customers are taking it all back two hours later.

“They’ll go, ‘This is beautiful. This is fantastic. This is great. I don’t know what my friends were talking about,’ ” Rahlmann says. “And I say, ‘Spread the word. Tell them. Let them know how fantastic it is.’ ”

Rahlmann says the gig brings him closer not just to his passengers, but the rest of the city, too. He has become friendly with everyone from Lombard Street’s landscaper to the Fort Point surfers. And nine years into his tour guide business, he’s still curious, still learning new things.

Those are the days when the work feels less like Rahlmann’s job, and more like a calling.

“I love sharing history. I love showing people the city,” Rahlmann said. “I love having them leave not only just seeing sights, but having a sense of what makes San Francisco the special city that it is.

“Trying to give them a sense of why we all came here in the first place.”

Reach Peter Hartlaub: phartlaub@sfchronicle.com; Twitter: @PeterHartlaub