The U.S. economy was "treading water" in the third quarter, according to an inaugural index of economic strength from Fitch Ratings Inc. The firm said in a news release that its latest index is not related to its warning Tuesday that it might downgrade the country's debt rating. Instead, its first measure of the U.S. economy is based on an evaluation of economic drivers such as mortgage and credit card performance, corporate defaults, gross earnings and forecasts for capital expenditures. On a scale that ranges from 10 to -10, the U.S. economy registered a zero in the third quarter, according to what the company calls the Fitch Fundamentals Index.