The U.S. budget deficit is likely to fall by $60 billion in 2015 due to strong revenue gains, the Congressional Budget Office said on Tuesday, enabling the government to stave off default without a debt limit hike perhaps through early December. The CBO said it now estimates a $426 billion deficit for fiscal year 2015, down from its $486 billion forecast made in March. The new forecast would bring the deficit to its lowest dollar amount since 2007, and as a 2.4 percent share of U.S. economic output, it would be below the 50-year average.