TL;DR: Brisbane diesel swings about 4.5 c/L in a typical month. Petrol swings about 45 — roughly ten times as much. Petrol runs a manufactured discount cycle; diesel just tracks wholesale, moving about cent-for-cent with it. So there’s no diesel trough worth waiting for. For a diesel tank the saving is in which servo, not which week. At the median, diesel also runs about 10 c/L dearer than petrol (189.9 vs 179.9 c/L over this window).
One of you put the whole thing in a review of the other app:
“in my city, diesel is on a different cycle making this app almost useless for estimating when to fill up my second vehicle.” — Google Play 4★
And another had the reason:
“Diesel is used by industry a lot, so there’s no incentive to cycle the pricing… the consumer/retail market for diesel is a lot smaller.” — r/CarsAustralia
That second bloke is basically right. Here’s the data behind it.
The claim
Diesel doesn’t ride the petrol price cycle. So the cheap-Tuesday folklore, the “wait for the trough” advice, the mental scoreboard of where the cycle sits — none of it maps onto a diesel tank.
What we checked
Brisbane metro, stations within 30 km of the CBD (473 in the set), from 15 January 2025 to 18 July 2026. We rebuilt daily price levels for petrol (ULP) and for diesel, then measured four things: how far each fuel swings in a typical month, how big its day-to-day jumps are, how tightly each one tracks Brisbane wholesale — the Terminal Gate Price, or TGP, the price the fuel leaves the terminal at before the servo’s own margin — and what margin the servo puts on top.
Two method notes, because they change the answer:
- Diesel means the diesel family — standard diesel and premium diesel unioned per station. In the metro, plenty of stations report their diesel under the “premium diesel” code, so reading one code alone gives you a wrong, sparse answer. Combining them is deliberate, and it’s what the app does too.
- We built a daily price panel, not a pile of change-events. For each station we took its end-of-day price and carried it forward across every day, then took the median across the stations reporting that day — 434 to 440 for ULP, 463 to 473 for diesel. That’s the same method our earlier posts used to measure the petrol swing, so we’re measuring like with like, and both land on 430-plus stations reporting daily.
One window caveat, stated up front so nobody thinks we cherry-picked the calm months: there’s a data gap from 1 March to 27 April 2026 where reports went sparse, and we excluded it everywhere — carrying prices forward across it would have invented a fake ramp to about 259.9 c/L that never happened at the bowser. Everything below is on the rest of the window.
What the feed shows
Monthly swing — the headline. Take each near-complete month (at least 20 panel-days) and measure how far the daily median travelled from its cheapest day to its dearest. Across 15 such months, the median month looks like this:
- Petrol (ULP): 45.0 c/L trough-to-peak — the same ~45 c/L both our earlier posts land on.
- Diesel: 4.5 c/L (the middle of the months; the spread runs from 3.0 to 9.0).
Petrol breathes about 45 cents a month. Diesel barely moves 5. Diesel’s swing is about one-tenth of petrol’s — petrol’s discount cycle is roughly ten times deeper.
Day to day. The single biggest one-day jump in the metro median over the whole window was petrol +41.0 c/L — that’s the cycle snapping back to the top in a morning, the “rockets up” drivers rage about. The biggest diesel one-day jump was +7.0 c/L. On a typical day the petrol median moves 1.0 c/L; the diesel median moves 0.0. Petrol lurches; diesel drifts.
Does diesel track wholesale? Yes — and this is the real tell. Two ways to check it, and they agree.
On the raw price level, diesel retail and Brisbane wholesale move almost lockstep across the window (correlation 0.94). Petrol retail barely relates to wholesale at all (0.13) — its price is set by where the cycle sits, not by what the fuel costs.
Level correlations can ride a shared long-run trend, so we also stripped the trend out and looked purely at week-to-week moves. Same story, sharper: week to week, diesel retail moves with wholesale (0.80), while petrol retail barely does (0.02), because week to week petrol is driven by the cycle, not the cost of the fuel. For every cent diesel wholesale moves, retail diesel moves about a cent back — a roughly one-to-one pass-through. Petrol retail swings about two and a half times wider than its own wholesale ever does, because the cycle is layered on top.
You can see it in the margin over wholesale too. Diesel’s retail margin sits in a stable band — a median of 22.7 c/L, tight (it wobbles by about 5). Petrol’s margin is the cycle: a median of 16.0 c/L but swinging more than three times as wide (it wobbles by about 18) as the servo drifts off wholesale at the trough and piles back on at the peak.
The verdict
Petrol has a manufactured retail cycle — a sawtooth the majors run on top of wholesale. Diesel doesn’t. Diesel is priced close to what it costs to buy, moves when that cost genuinely moves, and otherwise sits fairly still. There’s no discount trough because there’s no discount game.
And to be fair to diesel, it’s not that the price never moves — it’s that it only moves when wholesale genuinely moves. Through 2026 the diesel wholesale price climbed, and diesel retail climbed with it, cent for cent. That’s a real cost passed through, not a two-week retail game. When the barrel goes, diesel goes with it; there’s just no trough to wait for on the way.
So what does that mean for your tank?
If you run a diesel — the ute, the rig, the van — stop trying to time the petrol cycle. There’s no reliable dip to wait for and no peak to dodge. Your saving isn’t in timing, it’s in distance: on any given day the spread between servos is bigger than diesel’s whole monthly swing.
Do the maths at the pump. Diesel’s typical monthly swing is 4.5 c/L, so even if you perfectly timed the cheapest and dearest day of a month, a 90-litre fill would be worth about four dollars — and you’d need to fill on exactly the right day to get it, which nobody does. The servo across the suburb that’s 15 c/L cheaper today is worth more than three times that, right now, no waiting. One driver on the old app already ran the numbers:
“Fuel 2km from me is 157.9/L, yet if i drive my diesel ute 12km down the road… saved me $9.80 for 70L of diesel.” — Petrolspy Google Play 5★
That’s the game for diesel: the cheapest servo near you today, not the calendar. That’s what fill.pinly.com.au is for — pick diesel, see the spread across the map, drive to the cheap one. The price you see comes straight from the Queensland Government feed, the price each station reported it was charging, not a stranger’s guess at the bowser.
One honest limit
These are Brisbane-metro medians, and a median hides the edges. Some independents run wider diesel swings than 4.5 c/L; the “diesel family” figure blends standard and premium diesel, so your particular grade at your particular servo can sit off the middle. The margin numbers put a Brisbane-region wholesale benchmark under a 30 km metro retail panel — they still carry freight, excise, GST and the servo’s own costs, so read them as an indicative gross-margin gap, not profit. And the 4.5 c/L is a typical-month figure: zoom into 2026 alone and diesel’s monthly swing looks bigger, near 16 c/L — but that’s the diesel wholesale price genuinely climbing and retail following it cent-for-cent, not a discount cycle you could have timed. The wholesale-tracking result is the point: when diesel moves, it’s the cost moving.
Scope: Brisbane metro (473 stations within 30 km of the CBD), ULP (petrol) versus the diesel family (standard + premium diesel), Queensland Government price feed, 15 January 2025 – 18 July 2026, with the 1 March – 27 April 2026 data gap excluded. Cycle amplitude is the median across near-complete months (at least 20 panel-days) of that month’s daily-median trough-to-peak swing — the same method our earlier posts used for petrol. Wholesale is the Brisbane Terminal Gate Price, matched by fuel type on its published business days; correlations use the 353 aligned business-day matches.