Forbes magazine reported in April that Slim had overtaken billionaire investor Warren Buffett for the No. 2 spot in the world's richest stakes but was still behind Gates.
Mexico has a huge rich-poor divide, with a tiny elite holding most of the country's wealth and around half the population living on less than $5 a day.
Forbes bumped up Slim because gains from his holding company Carso and fixed-line telecom Telmex added to the Mexican's fortune while shares of Buffett's Berkshire Hathaway fell in the same period.
Three months ago, Sentido Comun's Garcia begged to differ with Forbes and calculated Slim's wealth as more than Gates' -- but only by a whisker. Now he says there is no doubt whose fortune is bigger at current share values.
"When I put Slim ahead three months ago Forbes bumped him up to second place (in world rankings) a few days later," Garcia, also the publication's editor-in-chief, told Reuters. "Let's see if the same happens again."
Spokespeople at Forbes magazine were not immediately available for comment.
Garcia, who uses Forbes' calculations for U.S. billionaires' wealth, says the 5.7% increase in Microsoft share prices in the second quarter is no match for the sharp rise in valuations of Slim's companies.
Shares of Telmex in the second quarter rose 11% and Slim's bank, Inbursa, saw its stock advance 20 percent.
Garcia's Sentido Comun, which translates as "common sense," reckons Slim and his family own a fortune equivalent to 8% of Mexico's gross domestic product.
For Gates to be worth 8% of the U.S. economy, his fortune would have to grow to more than $13 trillion, 17 times his current wealth, according to Sentido Comun.
Slim, known for his Midas touch in turning around struggling businesses and turning them into profit-making machines, told Reuters in an interview this year he was not in the habit of calculating his fortune on a regular basis.
Slim and his chief spokesman Arturo Elias Ayub were not immediately available for comment.