Costco Wholesale, petrol and diesel

There is no price series for Costco Wholesale. A brand line is drawn only where at least 50 of its forecourts published a price at that reading, and Costco Wholesale has 18 in the archive altogether. A median over a handful of sites, plotted as a line and labelled with a chain's name, reads as a statement about the chain. It would be a statement about those few.

How often Costco Wholesale changes its prices is not here. We read every six hours, so a change made and reversed inside that window leaves no trace. Any frequency would undercount.

The gap to the national median is not a policy. Brands sit in different places — motorway sites, city sites, supermarket forecourts — and the mix moves the median on its own. This page compares what was published, and stops there.