TL;DR: Across Brisbane metro, the independents (United, Liberty, Metro) average 12.3 c/L under the majors on E10 — but the median day is just 7.0 c/L, and the gap only lands in the claimed 15-25c band on 12.5% of days. Costco is stronger — 20.4 c/L average, 14.2 c/L median. The whole spread is a cycle-peak effect, not a permanent edge.
The claim
You’ve seen the line on Whirlpool:
“You guys should really check out Costco Petrol. They’ve consistently been 15-25c cheaper than the big petrol chains.” — a Whirlpool poster
It gets repeated about the independents too — United, Liberty, Metro. It’s stated as an everyday fact. So I tested it as one.
What I checked
Station-level, in-house data only, on the QLD Government feed — no user submissions. I built a daily price panel across the 473 in-range Brisbane-metro stations (within 30km of the CBD) from 15 January 2025 to 22 July 2026, carrying each station’s last reported price forward day by day, then took the median price for three brand families each day. E10 is reported by a subset of those stations, so each group median below is drawn from the stations in that group that report E10, not all 473:
- Majors — the big retail chains, not major refiners specifically: BP, Caltex, Ampol, EG Ampol, Shell, Reddy Express, 7-Eleven (204–300 reporting E10 daily; the count swings because not every major site sells E10 or reports it every day — grade-reporting churn moves the daily tally by up to 100 stations).
- Independents — United, Liberty, Metro Fuel (~40 stations).
- Costco — the two Brisbane-metro warehouses.
The comparison runs on E10, because Costco doesn’t report standard 91 in our feed — E10 is its headline unleaded, and the only fuel that lets it be compared at all. I re-ran the majors-vs-independents test on standard ULP and every number came back within about 1 c/L, so this isn’t a fuel-grade quirk. The daily gap is the major-family median minus each group’s median. The March–April 2026 data gap is excluded throughout.
What the data shows
Averaged over the ~496 days that remain once the March–April gap is excluded, both groups are genuinely cheaper — 12.3c for the independents, 20.4c for Costco. But an average hides the shape, and the shape is the whole story.
The independent gap only lands inside the claimed 15-25c band on 12.5% of days. It’s above 15c on 23.9% of days — roughly one day in four — and past 25c on about one day in nine (11.4%), only at the sharpest peaks. The typical day is 7.0 c/L. At the cycle peak the gap runs around 27.0 c/L; at the trough it falls to 5.0 c/L. That’s a 5x swing driven mostly by the majors ramping while the independents stay flatter.
Costco is a stronger case but the same shape. In-band on 15% of days, above 15c on 49.2% of days — about one day in two — with a median of 14.2 c/L. At the peak it runs 43.3 c/L under; at the trough, 3.8 c/L. Costco routinely clears 25c — about one day in three — while the independents only get there at the sharpest peaks.
Verdict
The myth as literally stated — 15-25c every day — is false. It’s true near the cycle peak and nowhere else. The independents beat 15c about one day in four and clear 25c only about one day in nine, at the sharpest peaks; Costco beats 15c about one day in two and is the only group that routinely clears 25c — about one day in three. The gap isn’t a permanent structural edge. It’s the majors’ ramp measured against flatter pricing, biggest exactly when you’re most tempted to panic-fill.
So what, for your tank
Don’t wait on a servo because someone told you it’s “always 15c cheaper.” On a random Tuesday the independent down the road is more likely 7c under than 20c, and near the trough everyone converges to within a few cents. The edge is real when the majors are high — which is the worst time to be buying anyway. The better move is to fill near the bottom of the cycle at whoever’s cheapest that day, major or not, and check the actual number before you drive. You can see today’s spread near you on the Pinly Fuel map. If you want the timing side of this, how the Brisbane price cycle works covers where the cycle sits.
The honest limit
Costco here is two warehouses — a median of two prices, not a population, and it’s membership-gated, which we don’t price in. Treat its figures as indicative. And Costco can only be compared on E10, since it doesn’t report 91; E10 runs about 2 c/L under 91 at the majors too, though the benchmark here is the major E10 median, so that’s controlled for. The independent set is United, Liberty and Metro only. Everything traces to the QLD Government feed.
— Priya