Grey fleet can appear to be a relatively simple way to manage business travel. The organisation does not need to purchase or lease the vehicle and the employee uses their own car when a journey is required.
The cost, however, does not disappear. It moves elsewhere.
Every business mile can generate reimbursement costs, administration, compliance checks and reporting requirements. For vehicles provided through cash allowance schemes, the allowance itself may also form part of the wider cost of mobility. These costs are often spread across fleet, finance, HR and other departments, making the total cost of grey fleet mileage difficult to identify.
Grey fleet includes employee owned vehicles used for business journeys, personal lease vehicles and vehicles funded through cash allowance schemes. Although these vehicles sit outside the managed fleet, the journeys they make still create financial and duty of care responsibilities for the employer.
A recent Business Motoring poll found that two thirds of fleet professionals surveyed believe grey fleet is outdated. Yet research reported by Fleet News found that 81% of businesses rely on grey fleet for business travel.
That makes the issue less about whether grey fleet exists and more about whether organisations understand what it is actually costing them.
Why Grey Fleet Mileage Costs Are Easy to Underestimate
The most visible cost of grey fleet is the mileage claim.
For the 2026/27 tax year, HMRC's Approved Mileage Allowance Payment rate for cars and vans is 55p per mile for the first 10,000 business miles and 25p thereafter. As reported by Fleet News, the first rate increased from 45p to 55p in 2026, with the change applying from April.
AMAP defines the approved amount that can be paid without creating additional tax reporting requirements. It is not a compulsory reimbursement rate, although many organisations use it as the basis for their mileage policy.
For organisations that do reimburse at the AMAP rate, the cost can build quickly. An employee covering 15,000 business miles during the tax year would generate £6,750 in mileage payments: £5,500 for the first 10,000 miles and £1,250 for the remaining 5,000.
That is a substantial annual travel cost attached to a vehicle that may never appear within the organisation's fleet budget.
This is where grey fleet mileage can become misleading. There may be no lease payment or acquisition cost to draw attention, but the organisation is still paying for mobility every time the vehicle is used for business.
The Hidden Cost Sits Across Different Budgets
Grey fleet costs are rarely captured in one place.
Mileage payments may sit within employee expenses. Cash allowances may be processed through payroll. Licence and vehicle checks may consume fleet or HR resources. Insurance evidence and document renewals require administration. Finance teams process claims while sustainability teams may need the same mileage data for emissions reporting.
Individually, each cost can appear manageable. Together, they provide a much more accurate picture of what grey fleet travel costs the organisation.
This fragmentation also makes comparison difficult. A company vehicle has a visible monthly cost that can be assessed against utilisation, maintenance and other operating expenditure. Grey fleet may appear cheaper because there is no equivalent single figure.
Connected fleet cost management allows grey fleet mileage expenditure to be assessed alongside the wider managed fleet. Rather than judging grey fleet purely by reimbursement spend, fleet operators can compare the cost of different travel options against how often they are actually used.
For high mileage employees in particular, that comparison matters. Repeated mileage payments can eventually make a pool vehicle, hire vehicle or other managed option financially preferable.
Grey Fleet Duty of Care Creates a Cost of Its Own
Vehicle ownership does not remove the employer's responsibilities when somebody drives for work.
Employers are expected to manage the health and safety risks associated with work related driving, including journeys completed in privately owned vehicles. Appropriate arrangements are therefore needed to confirm that drivers are eligible to drive and that vehicles being used for work are safe, insured appropriately and legally compliant.
This creates an administrative requirement that does not exist simply because the vehicle is absent from the company fleet list.
Licence status must be checked. MOT validity needs to be confirmed where applicable. Appropriate insurance for business use needs to be evidenced and those records need to remain current.
Research reported by Fleet News found that over a third of grey fleet drivers had never been asked by their employer to provide evidence of a valid MOT. Gaps of this kind create both a grey fleet compliance issue and a potential financial exposure if an incident occurs while an employee is driving for work.
Effective grey fleet duty of care therefore has its own cost. Someone has to collect the information, validate it, follow up missing documentation and retain evidence that appropriate controls were in place.
Manual processes make that cost easy to overlook because it appears as staff time rather than a direct vehicle expense.
A structured grey fleet management process brings these checks into the same environment as the journeys and mileage creating the requirement in the first place.
Poor Mileage Data Adds Further Cost
The amount paid per mile is only part of grey fleet mileage management. Organisations also need confidence that the mileage being reimbursed is accurate.
Manual expense processes can make that difficult. Claims may be submitted late, mileage can be recorded inconsistently and approval processes can vary between teams. Changes to reimbursement policies or HMRC rates can add another layer of administration where calculations depend on spreadsheets or manual review.
The cost appears in several forms: time spent checking claims, corrections after processing, inconsistent approvals and limited visibility of how much business travel an employee is actually completing.
Accurate journey records also matter beyond reimbursement.
For organisations measuring business travel emissions, employee owned vehicle mileage can form part of Scope 3 reporting. Mileage that has already been captured accurately for expenses can therefore support another reporting requirement without separate reconstruction.
The value of good grey fleet mileage data is not simply knowing how much has been claimed. It is being able to understand who is travelling, how frequently, at what cost and in what type of vehicle.
When Grey Fleet Mileage Should Trigger a Different Decision
Grey fleet is often used because it provides flexibility. An employee needs to make a business journey and their own vehicle is immediately available.
That does not mean it remains the right option at every level of use.
An employee completing a small number of occasional journeys presents a very different financial case from somebody covering thousands of business miles each year. Yet without consolidated grey fleet reporting, both may continue under the same policy simply because nobody has compared the alternatives.
This is where mileage becomes a useful decision-making measure.
Fleet operators should be able to identify which employees generate the highest grey fleet mileage, how much those journeys cost and whether the pattern is temporary or recurring. The same analysis can be applied across departments, locations and journey types.
Connected fleet management reporting makes those patterns easier to identify. Instead of treating each mileage claim as an isolated expense, the organisation can assess the cumulative cost and determine whether another travel option would be more appropriate.
For employees receiving a cash allowance, the comparison should also consider the allowance alongside business mileage payments and the administrative cost of maintaining grey fleet compliance.
The aim is not to remove grey fleet automatically. It is to make sure the decision to use it remains financially justified.
Bringing Mileage, Driver and Vehicle Data Together
Controlling hidden grey fleet costs depends on connecting the information behind each journey.
A mileage claim identifies that travel has taken place. The driver record confirms who completed it. Vehicle information establishes what was used and whether it met company requirements. Compliance records demonstrate that the required checks were in place.
When these records are held separately, each business journey creates additional work to verify the complete picture.
Connected driver management allows licence information and grey fleet activity to sit against the same driver record. Mileage can then be considered alongside eligibility and vehicle information rather than treated purely as a finance transaction.
The same principle applies to grey fleet policy. Requirements around insurance, MOT status, vehicle standards and mileage claims are easier to enforce when they form part of the process an employee follows, rather than relying on a policy document and periodic manual checks.
This gives fleet operators a clearer view of both sides of grey fleet management: what each journey costs and whether the organisation has the controls in place to allow it.
Seeing the Full Cost of Every Business Mile
The hidden cost of grey fleet comes from fragmentation.
Mileage reimbursement is visible, but often separate from the cost of cash allowances, administration, compliance checks and reporting. Individual claims can appear reasonable while the cumulative cost of repeated business travel remains difficult to identify.
Effective grey fleet management brings those costs together. It gives organisations a clearer view of grey fleet mileage, the drivers and vehicles behind it and the wider obligations created by each journey.
That visibility also makes better decisions possible. Occasional grey fleet travel may remain the most practical option. High and recurring mileage may point towards a pool vehicle, hire arrangement or another managed alternative. The important point is that the decision is based on the full cost rather than the absence of a company vehicle payment.
Learn more about grey fleet management and how Prolius supports mileage, compliance and duty of care oversight for employee-owned vehicles used for business travel. To see how this works across a wider fleet operation, book a demo.