Ask ten UK drivers whether they brim the tank or trickle in a tenner at a time and you will get ten different theories. Some swear that a lighter car sips less fuel; others believe that fewer forecourt visits mean fewer impulse buys and a better shot at a cheaper price. With DESNZ weekly road fuel figures still hovering well above pre-2021 norms, the question is more than academic. So does topping up little and often actually cost more, or is it a rounding error dressed up as folk wisdom?
The weight argument, in perspective
Petrol weighs roughly 0.74 kg per litre, diesel about 0.84 kg. A 50-litre tank therefore adds around 37–42 kg when brimmed versus nearly empty. On a family hatchback tipping the scales at 1,400 kg kerb weight, that is under 3% extra mass, and only when the tank is completely full.
Independent testing by the AA and various motoring titles has repeatedly found that payload effects on fuel economy are small at typical car weights: in the order of 1–2% MPG penalty per 100 kg on mixed driving. Half a tank of fuel saved therefore buys you a fraction of a percent in economy — worth perhaps 0.2–0.4 MPG on a car returning 45 MPG.
Set against that, every trip to the forecourt involves a detour, idling in a queue and a cold restart. If your nearest cheap supermarket station is a two-mile round trip off your normal route, the fuel burned on that detour alone can wipe out any weight saving many times over.
Forecourt frequency and price timing
RAC Fuel Watch and the AA Fuel Price Report both track how quickly wholesale changes reach the pump. Retailers typically pass rises on within days but cut prices more slowly — the so-called rocket-and-feathers effect the CMA highlighted in its 2023 road fuel market review. In practice, the pump price on any given week can vary by 10–15p per litre between the cheapest supermarket and the dearest motorway service station.
That spread matters far more than tank weight. Filling 50 litres at 138.9p rather than 148.9p saves £5 in one visit. To lose that £5 to the weight penalty, you would need to drive many thousands of miles carrying an extra half-tank.
The practical implication: the driver who fills up fully at a consistently cheap station beats the driver who tops up £15 at whatever forecourt happens to be nearest. Little-and-often shopping tends to default to convenience pricing, and convenience is expensive.
When small top-ups do make sense
There are legitimate reasons to run with less fuel. Drivers on tight weekly budgets may prefer to spread spend across pay cycles. Anyone expecting a sharp price drop — for example ahead of a rumoured duty cut in an OBR-scored Budget — might reasonably delay a full fill. And company car drivers reclaiming mileage sometimes need clean receipts per trip.
There is also a case for not brimming a car that will sit unused for months, since modern E10 petrol can degrade in storage. The RAC and DVSA both note that E10 is more hygroscopic than the older E5 blend, so long-term storage warrants a slightly lower fill and a stabiliser.
For everyone else, the cleanest rule is: fill up when it is convenient and cheap, not when the gauge dictates.
A quick cost check
The table below compares two realistic strategies over a year for a driver covering 10,000 miles at 45 MPG. Prices are illustrative, drawn from the range DESNZ has reported through 2024.
| Strategy | Typical pump price | Fills per year | Detour miles/year | Annual fuel cost |
|---|---|---|---|---|
| Full tank, cheap supermarket | 138.9p/L | ~20 | ~40 | £1,403 |
| Full tank, mixed forecourts | 144.9p/L | ~20 | ~20 | £1,464 |
| £15 top-ups, nearest forecourt | 148.9p/L | ~65 | ~130 | £1,504 |
| Motorway services only | 158.9p/L | ~20 | 0 | £1,605 |
The takeaway
The weight of a full tank is a real but tiny factor — worth pennies, not pounds. The variables that actually move the annual fuel bill are the price you pay per litre and whether you make special trips to buy it. Filling up fully at a consistently cheap supermarket or main-road station, on routes you would drive anyway, beats trickling in tenners at whichever forecourt is closest. Little and often is not thrift; it is usually just paying convenience prices more times per year.
