Inflation is at 3.7%, but core is only 2.6%. Why is the Fed worried about a December hike?

The gap between headline and core comes from gasoline

September consumer prices rose 3.7% year over year, and core rose 2.6%. Most of the 1.1pp gap is energy. Gasoline rose 9.1% in a single month, and that increase accounts for more than half of the headline rise. Gasoline is 34.2% more expensive than a year ago (FT Portfolios, 2026-10-14).

Looking at core alone might seem reassuring, but the Fed looks elsewhere.

What the Fed worries about is second-round effects

An energy shock spreads through two channels. The direct channel is pump prices. The second channel is spillover into freight costs, airfares and service prices, which pushes core up.

• A Fed staff note estimates that second-round effects of past oil shocks were statistically significant but not large. For advanced economies, they raised fourth-quarter headline inflation by about 0.5pp (Federal Reserve FEDS Notes, 2023-12-15).
• A South African central bank study finds that core returns toward headline within a year, but only partially.

September core rose from 2.4% in August to 2.6%, with shelter up 0.4% on the month and airfares up 1.2% (FT Portfolios, 2026-10-14). This is the same reason the report saw monthly core of +0.3% as a weak sign of easing. It could be an early sign of second-round effects, but one month of data cannot confirm it.

What a December hike probability of about 74% means

Per the report, the CME probability of a December hike is about 74%, but the figure varies widely by source. This data is the last inflation reading before the October 28 FOMC. The Fed sees a hold in October as sufficient if core runs at around 0.2% a month, but one reading is that this would not end the debate over whether a hike is needed (BMO's Scott Anderson, pre-release forecast, 2026-10-14).

This time, core came in at 0.3% a month, above that threshold. That is why the market has priced in a December hike to a considerable extent. The 10-year yield falling from the 5.30% range in early October to 5.098% is also read as a sign that hike expectations are already priced in.

What investors should watch

• If oil stays near $110, October's headline inflation will rise again, and whether it spills into core will be the watershed for a December hike.
• If monthly core keeps topping 0.3%, the burden on growth-stock valuations grows. Conversely, if it returns to 0.2% or below, there is room for the hike pricing to unwind.
• If your portfolio is heavy in rate-sensitive semiconductors and small caps, it is safer to build exposure in stages rather than all at once before the October CPI release in mid-November.
• Energy and banks are relatively favored in this setup. The biggest risk, however, is a turn lower in oil prices.