NIM05705 | Class 1 NICs: Expenses and Allowances: Mileage allowances: Types of mileage allowances
From HM Revenue & Customs · National Insurance Manual
Mileage allowances may be based on a:
fixed mileage rate paid for each mile travelled which includes all of the expenses associated with running a car
fixed mileage rate paid for each mile travelled to cover only the cost of fuel, plus a separate payment for associated running costs
round sum allowance which has no reference to the amount of miles travelled or any other considerations
monthly or annual lump sum based on an estimate of the costs likely to be incurred in using the car
combination of the above.
Most employers pay mileage allowances for business travel only but some may also pay allowances for private miles.