WASHINGTON — Homeowners with adjustable rate mortgages tied to the one-year Treasury bill might not know the 42-year-old security died Tuesday. But not to worry, experts said.

        That's because the benchmark many banks use to determine the price of some adjustable rate mortgages is not based solely on one-year Treasury bill rates. The most popular method is based on the interest rate of all Treasury securities — from three-month bills to 30-year bonds.

        That index, called the one-year constant maturity treasury, will still be available to price some ARMs, but eventually won't include the one-year Treasury bill, the government has said.

        One-year Treasury bills became a victim of the burgeoning federal budget surplus, which made them obsolete. A final auction of the bills took place Tuesday.

        The Treasury Department will continue to use the one-year bill to calculate constant maturity treasury yields until Aug. 24, then drop it from the formula.

        “It will be pretty imperceptible to consumers,” said Doug Duncan, chief economist for the Mortgage Bankers Association.

        The association couldn't say how many homeowners have ARMs tied to the one-year Treasury bill, or what the mortgages are worth.

        The constant maturity treasury, published by the Federal Reserve based on daily calculations by Treasury, estimates what it would cost Treasury to borrow money at a given maturity on any day.

        Against the backdrop of growing budget surpluses, Treasury already had cut back sales of the one-year Treasury bills from monthly to every three months and also has eliminated sales of three-year and seven-year notes.

        The changes mean lower interest payments on the national debt, but also pose a challenge for investors because of the dwindling supply of Treasury securities, considered the world's safest investment.

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