Employees travelling for business with their private vehicle may be entitled to a mileage allowance, the rates of which are as follows:

The mileage allowance is not taxable and no social security contributions are deducted from it. 

The flat-rate mileage allowance that has been set at €0,4415 per km by the Federal Public Service Strategy and Support for the period from 01.07.2024 to 30.06.2025 inclusive is also considered a standard rate. Employers who opt for the application of the flat-rate system on an annual basis must adhere to it for the entire period from 01.07.2024 to 30.06.2025 inclusive. For this period, they cannot switch to the quarterly flat-rate system.

Source: https://bosa.belgium.be/nl/themas/werken-bij-de-overheid/verloning-en-voordelen/toelagen-en-vergoedingen/vergoeding-voor-0

https://www.minfin.fgov.be/myminfin-web/pages/public/fisconet/document/9fe9bf86-08aa-485a-9dff-5573412b0cdc