TL;DR: For the price per litre, the size of your fill makes no difference — fuel’s sold by the litre, so 20 litres or 60 both cost you the same rate. Today the cheapest Brisbane servo is 191.5 c/L, about 16c under the 207.9 median — roughly $9 on a 55-litre tank, just for picking the right servo. Across a cycle the swing runs about 42c a litre, roughly $23 a tank, depending only on which week you fill. Fill size 0c, servo 16c, week 42c. Chase those two, not the size.
I get this one across the op-shop counter most weeks, usually from someone counting coins. “Am I better off putting a bit in whenever I pass, or waiting and doing one big fill?” It feels like it ought to matter. It doesn’t, not the way people think.
The question
“Is it cheaper to keep topping up a little at a time, or do one big fill?”
The short answer
For the price per litre, neither. Fuel is priced per litre, so the servo charges you the same rate whether you buy 20 litres or 60. Twenty litres at 200 c/L is 20 times the rate; sixty litres is sixty times the same rate. The size of the fill is a rounding error — it changes the total on the receipt, not the price on the board.
So the honest answer flips the question. The two things that actually move your fuel bill are where you fill and when — and both of them dwarf the size of the tank.
The three levers, in cents
Here’s the same litre, seen three ways. Same feed, same day, same city.
Where — about 16c, today. Right now the Brisbane metro median is 207.9 c/L and the cheapest in town is 191.5 — a gap of about 16c a litre. On a 55-litre tank that’s roughly $9 you keep, today, just for filling at the cheap servo instead of the middle one. No waiting, no discipline — you just pick the right board. (If the cheap board’s a suburb away, the drive almost never eats the saving — I’ve done that sum in how far it’s worth driving for cheap fuel.)
When — about 42c, across the cycle. Brisbane prices ride a cycle. Back on 12 July the trough sat near 167.9; the peak lands near 209.9. That’s a swing of about 42c a litre depending only on which week you happen to fill. On the same 55-litre tank that’s roughly $23 — more than double the servo saving, and it costs you nothing but a bit of timing.
Where the two traps hide
Neither “little and often” nor “one big fill” is wrong by itself. The traps aren’t the fill size — they’re the habit each one drags along.
The trap with topping up on empty is that it forces the fill. If you run the tank near dry every time, you buy whenever it runs out — and that’s often at a bad price, because you have no choice but to fill today. The trap with one big fill is only that you might do it at a peak, and then you’ve locked in 60 litres at the worst rate of the month.
So what for you
The rule that falls out of this is short:
- Keep enough in the tank that you’re never forced to fill on empty at a peak. Running-on-fumes is what hands the servo the timing, and the timing is worth about 42c. (I’ve run the wait-for-the-trough versus fill-on-empty numbers — the discipline is worth about $100 a year, but one fill caught at the crest wipes out a month of it.)
- Do your big fills when the cycle’s low, and at a cheap servo. That’s both levers at once — the 42c and the 16c on the same tank.
- Top up small if you’re caught high. A few litres at a peak just gets you to the next trough; no point loading 60 at 209.9.
The size of the fill was never the game. It’s the where and the when. Both of those are on the Pinly Fuel map — the cheapest servo near you right now, straight off the Queensland Government feed, so the number you see is the number at the pump.
How we measure: Brisbane-metro prices from the QLD Government feed, 9 September 2026 — median 207.9 c/L, cheapest 191.5, a gap of about 16c. The cycle swing (about 42c) is the trough near 167.9 on 12 July to the peak near 209.9; a cycle read a few weeks either way shifts it by a couple of cents, not the shape. Dollar figures assume a 55-litre tank — scale them to yours; every 1c of gap is about 55c a tank. Fill size makes no per-litre difference because fuel is priced by the litre; it changes the total, not the rate. This is the metro median for ULP/91 — not premium, E10 or docket discounts.