September 2026 CPI preview: Weekly fuel prices through September 21 point to renewed energy pressure ahead of the official inflation report, even as market sentiment appears optimistic. This is an early warning from pump prices, not a numerical forecast of September CPI. Gasoline prices enter the Consumer Price Index (CPI) directly; diesel can affect the cost of moving goods. Rising mortgage rates also squeeze prospective borrowers, although mortgage interest itself is excluded from CPI.
1. Energy: gasoline and diesel climb again
The U.S. Energy Information Administration (EIA) reported national average retail prices of $4.478 per gallon for regular gasoline and $6.529 for on-highway diesel for the week of September 21. The three September weekly readings available so far average $4.318 and $6.260 respectively.
| Fuel | August 2026 average | September through Sept. 21 | Change vs. August | Change vs. Jan. 5 reading |
|---|---|---|---|---|
| Regular gasoline | $4.058 | $4.318 | +6.407% | +54.435% |
| On-highway diesel | $5.462 | $6.260 | +14.616% | +80.050% |
ATN calculations from EIA weekly observations. The final column is a comparison with the first weekly reading of 2026, not a comparison between two monthly averages. September’s average is incomplete and will change with later releases. Small differences may result from rounding.
The diesel increase matters beyond drivers: freight, agriculture and other diesel-intensive operations face higher costs that can work through supply chains. The EIA’s September Short-Term Energy Outlook points to tight distillate inventories and elevated diesel crack spreads, with a risk of further pressure during refinery maintenance and the fall harvest.
What did the latest published pump-price breakdown show?
The EIA’s gasoline pump-component history currently runs through May 2026. These shares provide context for the earlier rise in gasoline prices; they are not a breakdown of the September pump price. ATN has compared May across years and calculated the simple average of the five monthly shares from January through May:
| Component | May 2026 | May 2025 | May 2024 | 2026 Jan.–May average | 2025 Jan.–May average | 2024 Jan.–May average |
|---|---|---|---|---|---|---|
| Refining | 21.70 | 16.60 | 14.80 | 19.26 | 13.16 | 15.58 |
| Distribution and marketing | 14.80 | 17.20 | 17.50 | 12.16 | 16.88 | 13.78 |
| Taxes | 11.50 | 16.20 | 14.10 | 14.90 | 16.36 | 15.06 |
| Crude oil | 51.90 | 50.00 | 53.50 | 53.70 | 53.64 | 55.54 |
The January–May figures are unweighted arithmetic averages of the EIA’s monthly percentage shares, not the component shares of an aggregated five-month retail price. EIA rounds monthly percentages to one decimal place, so totals can differ slightly from 100%.
Refining’s share increased by 5.1 percentage points from May 2025 to May 2026. But to identify what drove the dollar increase at the pump, the component shares must be multiplied by each month’s retail price. EIA reports $3.150 per gallon for May 2025 and $4.479 for May 2026, a rise of $1.329. On the published rounded shares, the implied crude-oil component rose by approximately 75 cents per gallon and the refining component by approximately 45 cents. Crude oil therefore made the larger contribution to that May-to-May increase, while refining was also material. The remaining difference reflects distribution and marketing, taxes and rounding.
The EIA component labelled refining includes costs and profits. A higher share does not, by itself, establish how much individual refiners earned. Nor can the May breakdown tell us which component caused September’s renewed rise; more recent component data are needed for that conclusion.
2. Mortgages: a second squeeze outside the CPI headline
ATN’s average of the available September 1–21 daily readings for the Optimal Blue 30-year jumbo mortgage index is approximately 6.91%, versus about 6.69% in August. The corresponding 30-year conforming index is approximately 6.90%, versus 6.68% in August. Each has risen about 22 basis points on this month-to-date versus full-month comparison.
Using single-day readings to compare spreads consistently, the jumbo index was 7.026% on September 21 and 6.705% on August 31; the conforming index was 7.034% and 6.719%. The 30-year Treasury constant-maturity yield was 5.29% and 5.25% on those dates.
| Index | Aug. 31 spread | Sept. 21 spread | Widening |
|---|---|---|---|
| 30-year jumbo | 145.5 bp | 173.6 bp | 28.1 bp |
| 30-year conforming | 146.9 bp | 174.4 bp | 27.5 bp |
A wider spread makes new borrowing more expensive than Treasury yields alone would imply. It can reflect several factors, including mortgage-backed security pricing, rate volatility, funding costs, prepayment risk and lender margins. These data cannot establish that lenders widened spreads because they expect more nonperforming loans. Higher borrowing costs could nonetheless make new purchases, refinancing and housing turnover harder at the margin.
The Bureau of Labor Statistics explicitly excludes mortgage interest from CPI. Owner-occupied shelter is represented by owners’ equivalent rent, which is measured from rental data; higher mortgage rates do not mechanically raise it. Financing conditions may affect rents over time through housing supply and demand, but that is an indirect and uncertain channel.
September CPI preview: what to watch before the release
August’s official CPI report showed a 3.4% annual headline increase and a 16.3% annual increase in the energy index. The September EIA readings make another energy contribution to CPI plausible, but they cannot determine its size: EIA weekly national pump averages differ from the BLS price sample and CPI’s monthly calculation. September’s CPI result has not yet been released.
If fuel and financing costs stay elevated, households may have less room for discretionary spending, while firms may face higher delivery and capital costs. Businesses could respond by passing through costs, accepting lower margins or investing more in equipment and software to improve productivity. Effects on GDP and employment depend on how long the shock persists and how demand responds; they are risks, not predetermined outcomes.
What to watch next: weekly EIA gasoline and diesel prices; distillate inventories and refining margins; the mortgage-to-Treasury spread; and the September CPI release. A sustained rise across those measures would make inflation risk harder for an optimistic market to overlook. A reversal in fuel prices or narrower mortgage spreads would weaken the case.