U.S. Gas Jumped 9.5% and Diesel Added $1 — September 2026 Report

September at Costco went one way. Regular gasoline across the 569 U.S. stations we track finished at $4.60 a gallon — up 40 cents, or 9.5%, from $4.20 on September 1, after peaking at $4.66 on September 28. Diesel was worse: across the 63 warehouses that sell it, diesel rose $1.00 a gallon (+17.3%) to $6.82. Gas was the priciest ever for Labor Day and by late month said the national average was the highest ever for this time of year, with September tracking toward a monthly record. It is a sharp break from the near-flat V we logged in the August report; see how the same month played out in Canada, the UK, and Australia.

National averages of posted prices at tracked U.S. Costco stations, $/gal, for stations reporting on September 1 (stations added later in the month are excluded). Snapshot of calendar September 2026 — this article is not updated.

A month-long climb to $4.66 on September 28

There was no V this time. The national average rose on almost every day of the month: through $4.30 by September 7, a 6-cent single-day jump on September 11, past $4.50 by September 17, and through $4.60 by September 20. It topped out at $4.66 on September 28 and only then turned, giving back 6 cents in the final two days to close at $4.60. Premium tracked regular almost cent for cent, up 41 cents (+8.3%) to $5.28. Diesel never paused: it added a dollar in thirty days, peaking at $6.85 on September 28 before easing to $6.82.

The West led the climb; only Colorado fell

This was a broad rise. Of 569 stations, 539 finished dearer, 24 cheaper, and 6 unchanged. California did the heavy lifting: its Costco average jumped 76 cents (+14.4%) to $6.06, and the eight biggest station moves in the country were all Californian, led by Redwood City (+$1.10, to $6.46) and Pleasanton, Torrance, and South San Francisco (+$1.00 each). Nevada rose 72 cents, Hawaii and Utah 57, and Oklahoma 53 — the steepest climb in percentage terms outside the West at 15.7%. Colorado was the only state to fall, down 12 cents (−3.2%) to $3.78, with the Denver suburbs leading: Douglas County cut regular by 28 cents and Parker by 26. Georgia barely moved (+2 cents), which is how it ended up the cheapest state.

Biggest September movers by state-average price of regular at Costco, September 1 – 30, 2026. States with fewer than four tracked stations are left out.

The cheapest and most expensive states right now

The spread widened sharply. The gap between the cheapest and priciest state averages is now $2.45 a gallon— Georgia's $3.61 against California's $6.06 — up from $2.10 at the end of August. Georgia takes the cheapest-state spot from Indiana, which rose 47 cents (+15.0%) and now ties Texas at $3.64. Colorado's September dip lifted it into sixth-cheapest at $3.78. The priciest five are all Western, with Hawaii and Nevada now both above $5.20 and Idaho replacing Alaska. At the pump extremes, Fort Oglethorpe, Georgia, at $3.40 sits $3.20 below Eureka, California, at $6.60.

State-average price of regular across tracked Costco stations, September 30, 2026. Puerto Rico is excluded — its pumps price in dollars per liter.

Why prices climbed — and why diesel climbed twice as fast

Crude did most of the work. AAA put oil in the $90-a-barrel range over Labor Day and around $100 a barrel by mid-month, blaming continued volatility in the Strait of Hormuz. FleetOwner tied the late-month leg to attacks reported on September 17 and 18 — the same window in which the Costco average jumped 7 cents. By September 24, AAA said the national average of $4.48 was the highest ever for this time of year, with the month tracking at $4.30 against the previous September record of $3.83 from 2023 — even though early autumn usually brings relief.

Diesel is a supply squeeze on top of that. The national diesel average set a new all-time record on September 4, passing the 2022 peak, and kept going to $6.53 on September 22. The EIA expected U.S. distillate stocks to drop below 100 million barrels in September and stay under their five-year low through much of 2027, citing tight global distillate supply. That is why the 63 Costco diesel pumps we track rose 17% while regular rose 9.5%.

How Costco compares with the AAA average

The gap widened again. Costco's $4.60 versus AAA's $4.43on September 30 is a 16-cent spread, up from 12 cents at the end of August; AAA's own month ran from about $4.10 to $4.43, a 34-cent rise against Costco's 40. That is geography, not Costco getting more expensive as a retailer: more than a fifth of the warehouses we track are in California, where the club average rose 76 cents, and EIA's weekly survey had California regular up another 19 cents in the last week of September while the U.S. average edged down. Diesel shows the same tilt: Costco's $6.82 sits above AAA's $6.41 national diesel average, and EIA had California diesel at $8.18.

Will gas prices fall in October?

Probably a little, but from a high base. AAA's national average has eased from $4.48 a week ago to $4.41 on October 1, and the EIA's September outlook assumes Hormuz flows recover gradually, with Brent averaging around $90 a barrel in the second half of 2026. Iran has signaled it could reopen the strait within seven days under certain conditions, but nothing is settled. Note that the EIA outlook was finalized on September 3, before the mid-month spike; the next edition lands October 6.

Diesel has the least room to fall. With distillate stocks below 100 million barrels and heating-season demand arriving, the EIA expects tightness to persist well into 2027. And California remains its own weather system: at $6.06 the state's Costco average is 76 cents above where it closed August, and its prices were still rising when the national average had already turned. If you fill up there, the club discount matters more than the national trend.

Costco pump prices are a snapshot of calendar September 2026 (September 1–30) and will differ at the pump today. This site is independent and not affiliated with Costco Wholesale Corporation.