For series-I savings bonds, the September CPI-U numbers posted today are the final component of the inflation rate for new bonds issued starting November 1, and for old bonds in their next 6-month rate period. The CPI-U over the six months up to September 2023 increased by 1.9%, meaning the new inflation component will be 3.8%. I'm guessing that the fixed rate component, currently 0.9%, will increase to about 1.6%. That would make newly purchased I-bonds pay about 5.4% for their first 6 months.
I last bought I-bonds ~2 years ago when the inflation component was over 7%, a killer deal at the time. With their 0% fixed rate component, those bonds are now earning 3.4%, which will increase to 3.8%. Not good at all, since it's all of a sudden easy to get 5+% interest. I'm going to sell those old ones and maybe use the money to purchase new I-bonds with the higher fixed rate component. Not an obvious decision as it was 2 years ago. Depends a lot on what the new fixed rate really will be, unknown until Nov 1.