The U.S. Treasury today announced the new inflation-adjusted interest rate for I Bonds: 3.06% through April 2012. This was a dip from the 6-month rate of 4.6% for the preceding May 1 to Oct. 31 period.

Although 3.06% is a good six-month rate for a super-safe investment, I predict the Treasury won’t be selling a lot of I Bonds before Jan. 1. Why? Almost everyone interested in I Bonds bought to the maximum before Oct. 31, to capture the 4.6% rate for six months.

(The Treasury puts yearly limits on I Bond purchases. You can buy $5,000 in Treasury Direct and $5,000 in paper bonds. A couple can buy twice that. Paper bonds will no longer be issued after Dec. 31, except as a tax refund.)

Those May-to-October buyers will earn 4.6% for six months, and then 3.06% for six months, for a one-year  rate of return of 3.83%. I Bonds can be sold after one year (with a three-month interest penalty, or after five years with no penalty.)

So if a May-to-October buyer sells out after one year, they will have earned about 3.06% over the year. That is significant – and indicates the high desirability of I Bonds – because:

1. The best one-year bank CDs are paying about 1% right now and average 0.73%.
2. A one-year U.S. Treasury is paying a paltry 0.13%.

The new rate: Come Jan. 1, when most of us can buy I Bonds again, this 3.06% six-month rate is likely to look attractive. It means anyone who buys I Bonds before April 30 and holds them for the required one year will earn an interest rate of at least 1.53% over the year, even if they sell out after one year.

If you didn’t buy I Bonds up to the limit before Oct. 31, you missed an opportunity. Still, the new I Bond is preferable to any super-safe investment, including TIPS.

Here is Treasury Direct’s FAQ on I Bonds if you want to learn more.