TL;DR: The gap between what a diesel driver and a petrol driver pay in Brisbane has blown out to about 51 c/L — up from around 40 through late July and early August. The metro diesel median hit a fresh 252.9 c/L today, up about 62 cents since mid-July, a near-straight climb with no cycle. But the gap didn’t widen because diesel spiked: it widened because PETROL came off its 209.9 peak back toward 200 while diesel just kept grinding up. Diesel tracks wholesale on a two-week lag, so it never gets the retail cycle’s discount phase — there’s no cheap week coming for a diesel tank. The only lever left is which servo: the cheapest diesel today is 225.5 c/L (24Xpress Marsden Park, Ampol Foodary Waterford West), about 27 cents under the median.
I drive a diesel ute, so I hear this one a lot lately: “diesel’s a mile dearer than petrol now, what happened?” Something did — but probably not the thing you think. The gap between the two fuels used to sit around 40 cents. In the last fortnight it’s stretched to 51. And the reason isn’t a diesel shock. It’s that petrol had somewhere to fall and diesel didn’t.
The claim
Diesel is now about 51 c/L dearer than petrol in Brisbane, the widest this window. But read why carefully, because the intuitive story — “diesel jumped” — is wrong. The gap widened at the petrol end.
What we checked
Brisbane metro, using each station’s latest price reported to the Queensland Government feed. Diesel here means the diesel family — the “All Diesel” series, the cheapest diesel-family price per station, which is what a diesel driver actually pays at the bowser (plenty of metro sites file their diesel only under the premium code, so reading one code alone gives a wrong, sparse answer). I lined the metro diesel median up against the ULP (petrol) median week by week, and took the gap as diesel median minus petrol median. Today’s live pull is 500 stations: diesel median 252.9 c/L, petrol median about 200.9.
What the feed shows
The gap has walked from 26 cents to 51. Week by week, diesel-minus-petrol went 26.0c (12 Jul), 39.0c (19 Jul), 39.6c (26 Jul), then sat almost dead flat around 40 — 40.0c (2 Aug), 40.0c (9 Aug) — before jumping to 46.6c (16 Aug) and 51.0c (22 Aug). Three weeks of roughly 40, then a fortnight of blow-out. That’s the chart below, and it’s the spine of the whole post.
But watch which fuel actually moved. Through late July and early August both fuels were climbing together, so the gap held near 40. Then, in the second week of August, petrol’s cycle rolled over: the ULP median came off its 209.9 peak (5–9 Aug) and fell back toward 200, where it’s frozen at 200.9 today. Diesel did nothing of the sort. It kept grinding up — 249.9 on 9 Aug, a small wobble down to the mid-243s, then straight back up to today’s 252.9. So the gap widened because one line dropped and the other didn’t. Put the two side by side and you can see the fork.
Diesel is up about 62 cents in six weeks — a near-straight climb. It started this window at 190.9 on 11 July and has walked up almost every day since: 243.9 on 15 Aug, then 246.5, 246.5, 247.9, 249.5, 249.9, 251.5, 251.9, and 252.9 in today’s live pull. On the weekly median it reads 191.9, 199.9, 219.9, 233.9, 246.9, 244.9, 249.9 — a rise, not a sawtooth. There’s no trough in there because there’s no cycle to make one.
Why diesel doesn’t get a cheap week
This is the mechanical heart of it, and I’ve measured it before so I’ll keep it short. Petrol runs a manufactured discount cycle — a sawtooth the majors hoist to a peak then drift down from. Diesel doesn’t. Diesel tracks wholesale, not the cycle — it moves with the Terminal Gate Price, the price the fuel leaves the terminal at, on roughly a two-week lag. So when wholesale climbs, diesel climbs, cent-for-cent, and just keeps going until wholesale turns. It never gets a discount phase because there’s no discount game to play.
That’s the whole asymmetry in this fortnight. Petrol reached its peak and the cycle pulled it back down — the trough Dan was waiting on this lap barely showed, but petrol still gave back nine-odd cents off the top. Diesel had no top to fall from and no cycle to pull it back, so it kept climbing. Petrol fell, diesel didn’t, and the gap opened up.
(If you’re wondering how this differs from the diesel-up-52-cents post from July: that one was about diesel’s own rise and what drove it — excise, crude, cycle timing. This one isn’t about diesel’s rise at all. It’s about the gap to petrol, and why it blew out this fortnight even though diesel just kept doing what diesel does.)
What it means for your fill
- Stop waiting for a diesel “cheap week”. There isn’t one coming. Petrol drivers can hold out for the cycle to roll over; diesel drivers can’t. The diesel line only turns when wholesale turns, and right now it’s still pointing up.
- The gap is not a diesel problem to solve — it’s a petrol thing that happened. Don’t read 51 cents as “diesel went crazy”. Diesel did the same grind it’s done for six weeks; petrol just stepped down off its peak.
- Your only lever is which servo, and it’s a big one. The cheapest diesel in the metro today is 225.5 c/L — 24Xpress Marsden Park and Ampol Foodary Waterford West — with 7-Eleven Marsden at 226.9 and Liberty Bellbird Park at 229.5 close behind. That’s about 27 cents under the 252.9 median. On a 130 L ute, 27 cents a litre is roughly $35 a fill, no waiting required.
- Shop the map, not the calendar. Pick diesel on fill.pinly.com.au, see the spread across the metro, and drive to the cheap one. Every price there is the one that station reported to the Queensland Government feed — not a stranger’s guess at the bowser.
The short version: the gap blew out to 51 cents, but not because diesel spiked. Petrol fell off its peak while diesel kept grinding up on wholesale. There’s no discount phase coming to close it for you — so close it yourself by filling at the servo that’s 27 cents under the median today.
How we measure: Brisbane-metro stations within a box around the CBD, QLD Government price feed, last-known price carried forward to a daily median. Diesel is the “All Diesel” family — the cheapest diesel-family price per station, standard or premium, because many metro sites file diesel only under the premium code; it’s what a diesel driver actually pays. Petrol is regular unleaded (ULP/91). The gap is the diesel median minus the petrol median, weekly. Today’s live figures are from a 500-station pull; obvious data errors are excluded. This is a read of where the two fuels sit today, not a forecast — diesel turns when wholesale turns, and I can’t tell you the day.