The study, released Monday by Global Aviation Associates, Limited, says the officials overstated their estimates by as much as 150 percent.
The Washington D.C. based group was hired by the project's opponents. The firm predicts the expansion will create about 69,000 jobs and contribute about $7.3 annually to the economy.
That's about a third of the economic benefits projected by the city.
City officials dispute the firm's analysis. They say the project would create $195,000 jobs and infuse $18 billion into the economy each year.