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Sentido Comun said the Mexican billionaire's wealth had rocketed past Gates following the red-hot performance of his telecommunications firm, America Movil.
US-based Forbes magazine, renowned for its rankings of the world's wealthiest individuals, updated its listings in April to rank Slim as the second richest individual in the world, as he bested the legendary US investor Warren Buffett.
The Mexican financial website said Slim's lead over Gates amounted to billions of dollars.
'Substantially richer than Gates'
"Thanks to a 26.5% rise in the shares of America Movil during the second quarter, Slim, who controls a 33% interest in Latin America's largest mobile phone company, is substantially richer than Gates," Sentido Comun said.
"The difference between their two fortunes is around $9bn in favour of Slim," the financial website claimed.
It said it had based its calculations largely on the share price movements of companies controlled by Slim.
The website said soaring performances from Slim's other business interests had also helped propel him past Gates.
Aside from America Movil, Slim controls the INBURSA financial group and the Grupo Carso industrial firm with interests spanning retail stores, coffee shops and restaurants.
Business in his blood
One reason for Slim's meteoric rise might be because he is also still working. Gates stepped aside as Microsoft chief in 2000 to devote his energies to the philanthropic foundation he runs with his wife, Melinda.
Forbes in April had pegged Slim's wealth at a staggering $53.1bn, and said Gates was sitting on a $56bn fortune.
Slim, the son of Lebanese immigrants, has had business in his blood from his early days when he helped out in his father's shop, "The Star of the Orient".
The 67-year-old, who is heavyset and bearded, started out in real estate and was already affluent enough when he graduated from university with an engineering degree to buy stakes in a stock brokerage and a bottling firm.
During the crippling Latin American economic crisis of the early 1980s, Slim snapped up and reformed a number of distressed businesses, banking massive profits for Grupo Carso.
Savvy stock picks
Carso gained its name from the first three letters of Slim's name and the first two of his late wife's, Soumaya Gemayel.
Analysts say one of Slim's smartest and most lucrative deals occurred when he took control of Telefonos de Mexico (Telemex) in 1990 as the then government moved to privatise the sprawling monopoly.
Slim oversaw a $1.8bn investment to take over Telemex, but he then overhauled the company and expanded its service as the telecom firm became the star of the Mexican stock exchange and more than returned Slim's initial investment.
The Mexican billionaire has also made some savvy stock picks.
In 1997, he bought about 3% of Apple Computer at $17 a share shortly before the company launched its hit iMac computer. Twelve months later, Apple's shares topped $100.
Plastic watch
Despite his vast riches, Slim reportedly shuns corporate jets and flashy offices and sported a plastic watch during the 1990s.
Widowed in 1999, Slim has boosted his philanthropic presence and overseen his three sons' careers within his business empire.
Like Gates, he has developed a strong profile on the philanthropic front.
Earlier this month he allied himself with the foundation of former US president Bill Clinton and with Canadian mining magnate Frank Giustra to launch an anti-poverty campaign in Latin America.
Fifty-three percent of Mexico's population of 104 million live in poverty, which is defined as living on less than $2 a day, World Bank data show.
