The School-Run Line: Auditing Private Fuel in Mixed UK Fleets

Fuel cards are usually introduced as a cost-control measure. In a mixed fleet they can also become a tax trigger. A card that pays for a van's weekend trip or a car's school run is not simply a fuel purchase; it may be a benefit in kind. The difficulty is that cars and vans do not follow the same arithmetic, and a single monthly fuel report rarely separates the two.

Two rules in one fleet

For company cars, the fuel benefit charge is calculated using the vehicle's appropriate percentage and the fuel benefit multiplier set for the tax year. For 2026/27, the multiplier is £29,000. The appropriate percentage follows the car's CO2 emissions and fuel type. If an employer provides fuel for private use, the charge applies unless the employee makes good the full cost of that private fuel.

Vans are different. Where private fuel is provided for a van, a flat fuel benefit charge applies alongside the van benefit charge. It is not a CO2-based percentage calculation. That distinction matters because a fleet policy that treats every fuel card in the same way can create two separate payroll problems from one monthly fuel file.

The first control is therefore a vehicle-level flag. Each registration should be marked as car or van, and then marked again for whether private fuel is authorised. A blank field is not a neutral position; it is an uncontrolled one.

The data gap that creates the surprise

Most fuel-card reports are strong on litres, sites and dates. They are weak on purpose. A transaction can show that 48 litres were bought at a motorway services, but not whether the driver was travelling to a customer or to a family visit. That purpose has to come from a second stream: mileage and odometer records, supported by driver declarations or telematics where available.

The third stream is payroll. Payroll needs to know when private fuel was first made available, whether it was withdrawn, and what the driver repaid. A year-end scramble usually means these three streams have been kept apart for eleven months.

A monthly ledger that survives scrutiny

A workable ledger does not need to be elaborate, but it does need to reconcile at vehicle level. Each month should bring together:

• fuel-card transactions, litres and gross cost by registration;
• odometer readings at the start and end of the period;
• business and private mileage, with the source of the split recorded;
• private fuel cost, calculated from actual pump cost or a defensible cost-per-mile method;
• any repayment made by the driver, with date and amount;
• a payroll flag showing whether private fuel remained available.

The private fuel cost should not be estimated from a round sum. If the driver has used 120 private miles and the vehicle has returned 40 mpg, the fuel used is three gallons, or about 13.6 litres. At the pump price paid on the card, the cost can be stated. The driver should repay that full cost, not merely the tax that might arise. A partial repayment leaves a benefit in place.

For business mileage reimbursed through the payroll, HMRC advisory fuel rates provide a familiar benchmark. For private fuel repaid by the driver, the test is different: the employer needs evidence that the full private cost has been recovered. Using an advisory rate that sits below the pump price may not clear the benefit.

Opt-out and reimbursement compared

An opt-out approach removes private-fuel access. The driver pays for fuel personally and claims business mileage at the advisory rate, or the employer reimburses through an approved mileage scheme. The advantage is a cleaner benefit position: no private fuel is provided, so no car fuel benefit arises from the card. The cost is administrative effort and cash flow for drivers who fuel frequently.

A reimbursement approach keeps the card but requires monthly recovery. It can suit drivers who cover high business mileage and need the card for motorway purchases. The condition is discipline. Repayments should be taken promptly, not left until the following summer. HMRC allows making good by 6 July after the tax year, but relying on that deadline turns a monthly control into a year-end credit risk.

The choice should be costed, not assumed. Opt-out often reduces tax exposure but increases mileage claims. Reimbursement preserves convenience but needs a robust private-mileage declaration and a clear repayment method. Mixed fleets should test both against their actual duty cycles rather than applying one policy to cars and vans alike.

Controls that stop the year-end surprise

Three controls do most of the work. First, the fuel-card policy should state whether private fuel is permitted. If it is not, the card should be restricted or the driver should understand that private purchases will be recovered in full. Second, every driver should sign a monthly private-mileage declaration, with random odometer checks to test it. Third, payroll should receive a monthly exception report showing vehicles where private fuel was available but no repayment was recorded.

Telematics can help, but it is not a substitute for a payroll decision. Location data may show a journey outside normal hours; it does not by itself prove private use or calculate the benefit. The ledger still needs a human review and a documented conclusion.

The ledger is the policy

A mixed fleet cannot manage fuel benefit through a single card rule. Cars and vans attract different charges, and private fuel is a payroll event as much as a fuel purchase. The practical answer is a monthly ledger that joins transactions to mileage and then to payroll. Once that routine is in place, a small private-fuel habit is visible early, before it becomes a year-end tax surprise.