The Weight of a Bargain: Costing Tankered Fuel for UK Light Commercial Fleets

Fuel price is the most visible number on a fleet cost sheet. It is also the one most likely to be over-trusted. A van that fills at a cheaper forecourt and carries the difference back to base has turned cash into mass, and mass has to be paid for somewhere in the ledger. For UK light commercial fleets in 2026, the question is not whether tankering can save money. It is whether the saving survives the costs that sit behind the pump price.

The saving is simple; the cost is not

The apparent gain from tankering is straightforward: litres bought at the lower price multiplied by the price differential. A 50-litre top-up at 6p per litre cheaper appears to save £3.00. That arithmetic is correct as far as it goes. It stops short of the depot gate.

The costs that follow are smaller individually but they are not optional. They include the payload carried, the fuel used to move that payload, the driver time spent diverting, the fees attached to out-of-network card use, and the operational risk of an unfamiliar forecourt. A fleet that ignores these lines is not comparing pump prices. It is comparing one pump price with another while treating everything else as free.

Payload, weight and the legal margin

Diesel is not weightless. A litre is roughly 0.83 kg, so 50 litres adds about 41.5 kg to the vehicle. On a 3.5-tonne van that is rarely a dramatic figure, but it matters at the margin. Tools, stock, passengers and a full tank all compete for the same gross vehicle weight. A van running close to its limit does not have spare payload for a bargain.

The practical rule is to treat fuel as part of the load plan. If a vehicle is within 100 kg of its permissible gross weight, tankering should be excluded unless the fleet engineer has confirmed the axle loads and the operational need. The saving on a litre is unlikely to justify a weighing exercise, an overweight risk or a redistribution of stock.

The consumption penalty

Extra mass raises fuel consumption. The effect is modest for a light van and larger in stop-start urban work than on a steady motorway run. As a planning assumption, a fleet can use a sensitivity of 1% to 2% additional fuel consumption per 100 kg, with the lower end applied to motorway-heavy routes and the upper end to urban delivery cycles. The figure is not a precise engineering constant; it is a test the fleet can defend.

For a 50-litre top-up, the additional mass is about 42 kg. At 1.5% per 100 kg, the penalty is roughly 0.6% of fuel used while that fuel is carried. On a 300-mile week at 30 mpg, the base consumption is about 45 litres. The penalty is therefore a fraction of a litre: pence, not pounds. On its own, weight rarely kills a tankering case. It becomes relevant when combined with driver time and when the vehicle is already working hard.

Driver time and diversion cost

Driver time is usually the largest hidden cost. A forecourt that is genuinely on the route costs almost nothing in time. A forecourt that requires a five-minute detour each way, plus queuing and payment, can absorb ten to fifteen minutes. At a fully loaded driver and vehicle cost of £25 to £35 per hour, ten minutes is £4.17 to £5.83 before any fuel is bought.

That cost has to be recovered from the litres purchased. If the driver buys 50 litres, a £5 time cost is 10p per litre. A 6p per litre saving is therefore a loss. If the driver buys 80 litres, the same time cost is 6.25p per litre, which turns a 6p saving into a marginal case at best. The size of the tank and the size of the fill matter as much as the price on the pole.

Card fees, reconciliation and route risk

Fuel-card fees can be fixed, percentage-based or hidden in out-of-network charges. A £1 transaction fee on a 50-litre fill is 2p per litre. A 1.5% fee on a £70 transaction is £1.05, or about 2.1p per litre. These are not hypothetical costs; they appear in the monthly reconciliation whether or not the site was convenient.

Route risk is harder to quantify but should not be ignored. An unfamiliar forecourt may have restricted access, a canopy height that does not suit a high-roof van, a queue that blocks a delivery schedule, or a nozzle that invites a misfuel. The fleet does not need to price every risk. It does need to decide which risks are acceptable and which are not.

A break-even test for the depot

The break-even pump differential is the total incremental cost divided by the litres saved, converted to pence per litre. In practice, a depot can use a simplified three-line test.

First, calculate the time and diversion cost in pounds. Add any card fee and a small allowance for the weight penalty. Second, divide that total by the litres that will actually be bought at the cheaper site. Third, compare the result with the pump differential.

A worked example makes the rule usable. A van takes 50 litres. The on-route cheaper site saves 6p per litre. The card fee is £1, the weight penalty is £0.30, and there is no diversion. Total incremental cost is £1.30, or 2.6p per litre. The 6p saving survives. The same van diverts ten minutes off route at a £30 per hour combined cost. The incremental cost rises to £6.30, or 12.6p per litre. The 6p saving does not survive.

For 2026, a sensible default is to require a differential of at least 3p per litre for a site on the planned route and at least 10p per litre for a site requiring a meaningful diversion. Those thresholds are not universal. They are a starting point that forces the depot to enter time, fees and weight into the same calculation as the pump price.

The depot discipline

Tankering works best when it is treated as a routed purchase, not a habit. The fleet should identify sites that are genuinely on established delivery lanes, agree a maximum extra fill of 40 litres or half the tank, whichever is lower, and exclude vehicles operating near their weight limit. Drivers should not be encouraged to top off beyond the agreed level; the marginal litre is the one most likely to be carried at a cost.

The half-tank question is therefore not a matter of frugality. It is a matter of marginal costing. A fleet that can show the break-even differential for each route has a defensible policy. A fleet that simply fills wherever the pole is cheapest has a policy that looks like savings and behaves like an unmeasured cost.