Summary
- How does the Treasury determine the I Bond fixed rate? It’s a mystery.
- A recent surge in 5-year real yields seems to justify an increase in the fixed rate, and 10-year real yields are also rising.
- Most I Bond investors are waiting until November 1 to complete their I Bond allocation for 2017. What are the odds of a higher fixed rate?
Treasury Inflation-Protected Securities 
Note, "when you establish an account..." It does say "at any time" (perhaps someplace else?) Regs are tricky!