Costco's latest growth numbers contain a useful contradiction: expensive gasoline made reported sales look stronger while depressing the company's gross-margin rate. A decline in fuel prices could therefore slow headline revenue growth without weakening the economics of the warehouse business.
In fiscal fourth-quarter 2026, comparable sales rose 9.4%, but the increase was 6.7% after excluding changes in gasoline prices and foreign exchange. The 2.7 percentage-point gap shows that fuel and currency contributed materially to the reported figure. Management said gasoline inflation alone lifted quarterly sales by about 3%.
September widened the difference. Costco reported 11.4% comparable-sales growth for the five weeks ended October 4, versus 7.6% after removing gasoline and currency. Net sales reached $30.02 billion, up 13% from a year earlier. A later Labor Day added slightly more than half a percentage point to the monthly comparison, another reason not to treat the headline as a clean run rate.
Why lower fuel prices are not simply negative
Gasoline is a high-volume, lower-margin business. Costco's fiscal fourth-quarter gross-margin rate was 11.02%, down 11 basis points from a year earlier, while management said the rate would have been 20 basis points higher without gasoline inflation. Cheaper fuel could subtract from reported revenue but improve the mix, because fewer sales dollars would come from a low-margin category.
The core business still looks healthy. Companywide comparable sales excluding gasoline and currency grew 6.7% in the fourth quarter and 7.6% in September. Shopping frequency rose 3.3% worldwide in the quarter, and the U.S. and Canada membership renewal rate reached 92.3%, according to the source report.
Valuation raises the standard
The source valued Costco at 45.6 times earnings versus 21.9 times for the S&P 500. Those figures are point-in-time estimates, but they illustrate the risk: investors paying a large premium may react badly to slower reported sales even if lower gasoline prices help margin quality.
The cleanest test is sales excluding gasoline and currency. If that measure stays near recent mid-to-high single-digit growth while gross margin improves, cheaper gas would change the optics more than the business. If traffic or renewal rates weaken at the same time, the sales slowdown would carry more information.