Is your employer paying you less than 45p per mile for your business mileage?
If the answer to the question is yes, then you will probably be able to claim a tax refund.
An example of how it works
If you receive 15p per mile from your employer and travelled 5,000 business miles in your personal car during the tax year, you will have received 5,000 x 15p = £750 in expenses.
However, HMRC's recommended rate of reimbursement is 45p per mile for the first 10,000 miles (and 25p per mile thereafter) and so you are able to claim the difference between the two rates as an expense against your salary.
It effectively reduces your salary by the difference, which in this case would be (45p - 15p) x 5,000 miles = £1,500.
If you were a basic rate tax payer you would be able to claim £1,500 x 20% = £300 back in tax and if you were a higher rate tax payer you would be able to claim £1,500 x 40% = £600 back in tax.
What to do next
If the difference between the mileage received from your employer and HMRC's recommended reimbursement is less than £2,500 then a claim can be made by simply filling out a P87 Form.
The difference in the example above is £1,500 and so a P87 Form would be the correct course of action.
If the difference is over £2,500, you will need to fill out a self-assessment tax return. In order to register for self assessment you will need to fill out an SA1 form.
Once you have completed the registration form it will take up to 6 weeks to process. You will then receive a letter from HMRC with your UTR number on it and you will then be able to submit your Tax Return online.