I often get asked by clients how to reclaim fuel and mileage when a car is used for both personal and business trips — and how to do it without waving a red flag in front of HMRC. It’s a common headache: business owners want to be fairly reimbursed or to claim legitimate expenses, but they don’t want to create more paperwork than the benefit is worth, and they certainly don’t want an avoidable enquiry. Below I share a simple, practical method I use with my clients that is HMRC-friendly, easy to maintain, and works whether you’re a sole trader, partner or director.

Two accepted approaches — pick one and stick to it

First, understand HMRC will accept either of these main approaches for mixed-use vehicles:

  • Approved mileage rates — use HMRC’s statutory mileage allowance (45p per mile for the first 10,000 business miles in the tax year, 25p thereafter for cars). This is the simplest for employees and sole traders: no need to calculate fuel bills if you reimburse at those rates.
  • Actual cost method — claim the business proportion of actual fuel and running costs (insurance, maintenance, depreciation, finance, road tax). This requires accurate records of total miles and business miles, plus fuel receipts or bank statements.

    • Choose one method per vehicle for the tax year (or employer reimbursement type). Switching between methods mid-year complicates things and can attract questions unless you have a clear reason and contemporaneous records.

    A simple, HMRC‑friendly method I recommend

    For many micro-business owners I work with, the best balance of simplicity and accuracy is this hybrid approach:

  • Use a short, contemporary logbook for a representative 12‑week period to establish business miles as a percentage of total miles.
  • After the 12 weeks, keep using a lightweight mileage log (app or paper) to record business journeys only.
  • At year‑end, claim the business proportion of actual fuel costs (total fuel spend × business mileage %) OR simply claim HMRC advisory mileage rate for business miles recorded — whichever gives the better, properly evidenced result.

    • This method is accepted by HMRC because the 12‑week contemporaneous logbook is an established way to estimate business use for a mixed‑use vehicle. It avoids needing to keep every single fuel receipt forever while still giving you a defensible basis for a business‑use percentage.

    How to run the 12‑week logbook (step‑by‑step)

  • Pick a representative 12‑week period — avoid holiday months where mileage drops to unusual levels.
  • Record total miles at the start and finish of the 12 weeks (from the odometer).
  • Record every business trip during the 12 weeks: date, purpose, start/finish mileage, and mileage for the trip. Don’t try to aggregate later — log contemporaneously if possible.
  • Save fuel receipts or note fuel spend during the same 12‑week period (or keep monthly fuel statements from your bank/credit card).
  • At the end of 12 weeks calculate: business miles ÷ total miles = business use %.

    • Once you have that percentage, you can reasonably apply it to fuel and running costs for the year (if you’re using the actual cost method). HMRC accepts a 12‑week sample as long as it’s representative and well‑documented.

    Practical example (table)

    Item Amount
    Total miles (12 weeks) 2,400
    Business miles (12 weeks) 960
    Business use % 960 ÷ 2400 = 40%
    Annual fuel spend (bank statements / receipts) £2,400
    Claimable fuel cost (actual method) £2,400 × 40% = £960
    Alternative: HMRC mileage (if 6,000 business miles this year) First 10,000 miles at 45p = 6,000 × £0.45 = £2,700

    In this example the HMRC mileage allowance gives a much larger benefit — but you must use the method you can evidence. If you’ve recorded 6,000 business miles, using HMRC rates is straightforward and HMRC won’t ask for fuel receipts to support a tax‑free reimbursement. If you claim actual costs, be prepared to show the 12‑week logbook and your fuel records.

    Record keeping — what HMRC expects

  • Contemporaneous logbook for the 12‑week sample (dates, purpose, start/finish odometer readings).
  • Odometer readings at the start and end of the tax year (or at the start/end of ownership).
  • Fuel receipts or bank/credit card statements showing fuel purchases; at minimum, a monthly reconciliation if you don’t keep every receipt.
  • For employers reimbursing directors/staff: records of payments, and evidence you used either the approved rates or actual costs and the basis for the calculation.

    • HMRC looks for consistency, reasonableness, and contemporaneous records. If your business‑use percentage is unusually high for the type of business, they may ask for more evidence — so keep detailed trip purposes and don’t rely on memory alone.

    Tools that make this easy

  • Mobile mileage apps: MileIQ, TripLog and Everlance are popular — they automatically track trips and classify business vs personal. I’ve seen clients save hours a month by using an app.
  • Fuel cards: companies like Shell and BP offer fuel cards that give you a monthly statement showing fuel purchases per vehicle — convincing evidence if you use the actual cost method.
  • Accounting software: Xero and QuickBooks let you attach receipts to expense claims and run reports that tie fuel spend to a vehicle or a user.

    • If you’re sensitive about privacy or just want minimal fuss, use an app that exports CSV or PDF reports you can store with your accounting system.

    Common red flags and how to avoid them

  • Claiming unrealistic business miles compared to the business type — be conservative and explain seasonal peaks with records.
  • No contemporaneous logbook — HMRC dislikes reconstructed records created months later.
  • Mixing methods: claiming HMRC mileage for some periods and actual costs for others without clear documentation.
  • Rounding mileage or expenses aggressively. Small rounding is fine, but consistent rounding up is a quick way to invite questions.

    • When to consult an adviser

    If your vehicle arrangements are complex — for example, multiple drivers sharing one car across business entities, directors using company cars with fuel provided, or highly mixed private/business use — get tailored advice. I’ve helped clients with company car fuel benefit calculations and director reimbursements to ensure payroll and P11D treatment are correct and that the business isn’t accidentally creating a taxable benefit.

    Finally, keep it simple: choose a defensible method, keep contemporaneous records, and use technology to minimise time spent on admin. That approach keeps HMRC happy and leaves you free to focus on running your business.