(FORTUNE Magazine) -- Carlos Ghosn circumnavigates the planet in his Gulfstream 550 once a month. He typically spends two weeks in Paris, ten days in Tokyo, and what's left of his time in the U.S. and the rest of the world.

But now the Nissan and Renault CEO - the first person ever to run two companies on FORTUNE's Global 500 simultaneously - has decided he doesn't have enough places to go.

At the invitation of financier Kirk Kerkorian, Ghosn has proposed an alliance between his two auto companies and General Motors (Charts) that could put him at the helm of the troubled industry giant. Add Detroit to the flight plan!

Putting one man in charge of three companies separated by thousands of miles in an industry as complex and competitive as automobiles is a bizarre prospect and at the moment still an uncertain one. The boards of Renault and Nissan backed Ghosn and have expressed wary interest in the proposal, under which each would buy 10% of GM's shares.

But their support is conditional on GM's management and board endorsing the plan. The current forecast is for a lengthy GM study of the arrangement, with no guarantee of a favorable conclusion.

Still, Kerkorian, who owns 9.9% of GM's stock, wants action. He is known to be unhappy with the pace of change under current CEO Rick Wagoner and has been prodding him to do more. Ghosn is equally impatient. He performed a historic turnaround of Japan's Nissan after taking charge in 1999, embarked on a similar effort with France's Renault a year ago, and has reportedly been considering new strategic options for both companies.

Despite GM's gargantuan problems - it lost $10.6 billion last year - Ghosn would find the challenge of fixing it appealing, particularly if he could benefit Renault and Nissan by doing so.

Even if Ghosn doesn't end up in charge at GM, it's useful to contemplate just what the world's most celebrated auto executive might change at the world's largest auto company. Some of GM's problems, such as its huge health-care and retiree obligations, seem relatively intractable. For the others, a playbook already exists - the history of the Nissan turnaround that Ghosn has taken pains to document. Here's the path he might take.

Critics love to point out that GM divides its 24% U.S. market share among eight brands, while Toyota, two-thirds GM's size and growing, makes do with three. Kerkorian advisor and GM board member Jerry York has publicly recommended getting rid of Saab and Hummer; others have advocated dumping Pontiac and Buick. But GM was pummeled for giving up on Oldsmobile a few years ago and could face billions of dollars in claims from dealers for effectively putting them out of business. Besides, engineering among the brands is so intertwined--Cadillac and Saab share a platform, as do Chevy and Hummer--that getting rid of one or more of them wouldn't necessarily save tons of money.

Super Carlos, as he is sometimes known, might have no qualms about killing all four brands, and perhaps Saturn and GMC too, as part of a big initial write-off. In return, he could offer dealers Renault franchises so that they could introduce U.S. buyers with Francophile tastes to such exotic models as Kangoo and Clio.

GM has suffered no end of embarrassment this year after launching gas-hungry SUVs and pickup trucks when a barrel of oil costs $75. The company's managers aren't quite as boneheaded as that makes them look--product decisions about this year's models were made three or four years ago, before fuel prices skyrocketed.

Nonetheless, Ghosn's Nissan was significantly more prescient. The company is coming out this fall with a new version of its Sentra small car and a new, even tinier car called the Versa. Ghosn, since he works in Tokyo and Paris, knows the value of small cars--he sees them parked on city sidewalks every day. Attractive new ones would go far in creating a more favorable image for General Motors.

Small cars would do for Ghosn what technology won't - boost fuel economy without costing a bundle. Being a numbers guy at heart, Ghosn is deeply suspicious of hybrid and alternative-fuel engines--expensive new technologies for which customers have so far displayed limited willingness to pay. He has invested only reluctantly in hybrid power trains at Nissan. So he would probably scale down GM's investment in hybrid trucks and eliminate entirely its space-shot effort to put a fuel-cell car on the road by 2010.

At Nissan, Ghosn used cross-functional teams to burrow into the company's innards, uncover waste and duplication, and improve productivity. He's told Kerkorian he can do the same at GM, locating billions of dollars in savings from synergies among the three companies. But he's going to have to look hard. GM has already made substantial progress in bulk buying and parts sharing by clustering its products worldwide around a few platforms. Lehman Brothers analyst Darren Kimball believes that savings from shared purchasing and engineering would be more than offset by organizational complexity and the stifling of creativity in a combined GM-Nissan-Renault. He thinks Kerkorian's plans have less to do with synergy than with "his desire to bring in someone he perceives is a more effective manager."

Public targets are anathema to Wagoner. Risk-averse at heart, he prefers to keep his cards close to his chest and reveals information only grudgingly. As a lifetime GMer, he has been embarrassed too often by having to explain why GM missed a profit or market-share target. Ghosn brags about his ability to set financial and performance targets three years out--and meet or exceed them. At Nissan he even offered to resign if he missed. How many other CEOs have done that?

Although still shiny, Ghosn's star has lost some sparkle this year as Nissan sales in Japan and the U.S. have slipped and Renault has issued profit warnings. "Ghosn is not a miracle maker," says one longtime industry observer. "He was merely a gaijin who did the unpalatable things [at Nissan] that a Japanese couldn't do." Some analysts view him as a turnaround specialist who is less effective at actually running a business.

But rescuing the ultimate basket case would make Ghosn nearly immortal. "It is very tempting for Ghosn to try to prove he can fix GM - what some may define as the ultimate auto-industry challenge," writes Goldman Sachs analyst Robert Barry. "Ego often trumps logic."

Then again, ego could also prompt Ghosn to take the sort of shockingly bold steps probably needed to get GM back on track. With his multicontinental monthly commute already beginning to wear, Ghosn, 52, may have his eye fixed on his legacy. What could be better than being remembered as the man who saved General Motors?

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