The longest decline in Treasuries this year has left U.S. government debt the cheapest since March 2011 when measured by real yields—and the best relative value, compared with German bunds, in more than two decades.

After inflation, 10-year U.S. notes yielded 0.91 percent last week, or 1.77 percentage points more than real yields on U.K. gilts, the widest spread in 25 months. Versus Germany, the securities are the least costly in 23 years when adjusted for the recent record-low interest rates around the world that distorted the normal relationship, according to FTN Financial.

Federal Reserve Chairman Ben S. Bernanke is counting on Treasuries to contain borrowing costs as the central bank buys $85 billion a month in securities to sustain the economic recovery that lifted U.S. consumer confidence to the highest in almost six years. The better relative yield for U.S. bonds may help bolster demand even as Warren Buffett said this month that he pitied fixed-income investors because of nearly record-low interest rates.