Any employer funded costs are deducted in full and business mileage is reimbursed at the HMRC defined advisory fuel rate for the users vehicle. The deduction and reimbursement are then combined to provide the payroll figure due for processing.
Example
Employer funded cost = £300
Business miles logged for period = 1700
AFR for vehicle logged against trips = 0.15p
1700 x 0.15p = £255
£300 - £255 = £45 deduction
If the reimbursement due exceeds the value of employer funded costs, a reimbursement will be processed overall.
Any employee funded costs are generally excluded from the calculation as cost of the fuel/electric is covered by the AFR x business mileage.
Personal miles are not included in the calculation, only business mileage and employer funded costs.