The US Bureau of Labor Statistics will release the September inflation report Wednesday morning, and this is the final piece of data needed to determine the inflation-adjusted interest rate for US Savings I Bonds from Nov. 1, 2014, to April 30, 2015.

I record these inflation numbers on my Tracking Inflation and I Bonds page. The new I Bond inflation-adjusted rate will be determined by the May to September increase in the Consumer Price Index for All Urban Consumers, otherwise known as CPI-U or ‘headline inflation’ in the media. The one twist from the media reports, though, is that TIPS and I Bonds use non-seasonally-adjusted inflation as the measurement. The index stood at 236.293 in May 2014 and rose to 237.852 in August (although it has declined for two consecutive months).

If you look at my Tracking Inflation page, you can see that inflation from May to August rose 0.66%. If inflation came in at zero for September, the new I Bond variable rate would be set at 1.32% (annualized) for six months. So once we see the September number at 8:30 a.m. Wednesday, we’ll know the variable rate for the next six months.

However … I Bonds are an odd investment because if you invest in I Bonds between now and Oct. 31, you will receive a fixed rate of 0.1% and a variable rate of 1.84% for six months and then you will receive the new variable rate for the next six months.

(Take a look at the chart to the right. It says I Bonds pay 1.94% through Oct. 31. That is the combination of the 0.1% fixed rate and the 1.84% variable rate, but if you buy before Nov. 1, you get that combo rate for the first six months. Get it?)

So if Wednesday’s inflation number comes in very low or negative, it might be smart to buy your 2014 allocation of I Bonds before Nov. 1 to lock in that higher variable interest rate for the first six months.

The other interesting thing about I Bonds is that the Treasury acts in total mystery when it sets the ‘fixed’ rate, which is currently 0.1% and never changes once you purchase the I Bond. We know nothing about the process the Treasury uses to determine this rate. However, my general guess is that the fixed rate tends to lag 70 to 100 basis points below the yield of a 10-year TIPS. See this chart.

A 10-year TIPS is yielding 0.28% today, so I would say there is a pretty good probability that the Treasury will lower the I Bond fixed rate to 0.0%, where it was from November 2010 to October 2013. But then again, the Treasury shocked everyone when it increased the fixed rate to 0.2% in November 2013, even with the 1o-year TIPS yield at just 0.40%. Six months later, the fixed rate dropped to 0.1% when the 10-year TIPS was yielding 0.49% … go figure.

My guess is that the Treasury either: 1) drops the fixed rate to 0.0% or 2) keeps it at 0.1% and there’s a near-zero chance of it increasing.

Purchase limits. I bought my 2014 allocation of I Bonds in February (and got the 0.2% fixed rate, which lasts the 30-year life of the I Bond). The Treasury allows individuals to buy $10,000 a year at TreasuryDirect.com, and an additional $5,000 in paper I Bonds issued as an income tax refund.

If you haven’t purchased your 2014 limit yet, you’ll have an interesting decision to make once the variable rate is set on Wednesday. Buy now or buy after Nov. 1? But you won’t know the new fixed rate until Nov. 1.