The biggest bond gains in almost a decade have pushed returns on Treasuries above stocks over the past 30 years, the first time that's happened since before the Civil War.

Fixed-income investments advanced 6.25 percent, almost triple the 2.18 percent rise in the Standard & Poor's 500 Index through last week, according to Bank of America Merrill Lynch indexes. Debt markets are on track to return 7.63 percent this year, the most since 2002, the data show. Long-term government bonds have gained 11.5 percent a year on average over the past three decades, beating the 10.8 percent increase in the S&P 500, said Jim Bianco, president of Bianco Research in Chicago.

The combination of a core U.S. inflation rate that has averaged 1.5 percent this year, the Federal Reserve's decision to keep its target interest rate for overnight loans between banks near zero through 2013, slower economic growth and the highest savings rate since the global credit crisis have made bonds the best assets to own this year. Not only have bonds knocked stocks from their perch as the dominant long-term investment, their returns proved everyone from Bill Gross to Meredith Whitney and Nassim Nicholas Taleb wrong.