When fuel is provided for a company car or van a separate income tax charge arises. The charge is applied regardless of whether the fuel is provided for private use, when an employer directly, or indirectly pays for the fuel for the vehicle or the employee is reimbursed for the cost of any fuel used.
However, the charge is intended to tax the fuel provided by the employer that is intended for the employees private use. Private mileage is any mileage that does not fall under the definition of business mileage. Business mileage does not cover the journey to the employees place of work. This aspect needs to be clearly defined when employees are reporting their mileage.
If an employer provides fuel for the employees private use, this is considered a benefit and therefore taxable, this tax charge is known as the Fuel Scale Charge. The tax on the benefit of free fuel is based on the benefit percentage derived in respect of the car of which a nominal value is applied. The government fuel benefit charge for calculating the tax rises to £24,500 in 2020/21.
If the employee 'makes good' the cost of the private fuel or the fuel is for business use only the fuel benefit would not be charged.
See Fuel Card Optimiser under Taxation