In order to allow certain users to access a specific mileage class, we will need to use tags:
Within Tag Types, create a Tag for the users for whom you want to access the specific vehicle class. (For example, you might create a Tag Type called 'Car Type' and within it, create tags such as 'Personal Usage' or 'Company Car').
Then create a custom mileage class and under settings, where it says 'Available for' insert the tag you just created.
Go to Settings then Users and choose the user then click on their name to edit.
In their settings, under Tags, enter the Tag in the relevant Tag Type.
When the user goes to + Add Vehicle (on the right hand side of their user settings screen), they will be given the option to choose the new mileage class associated with their Tags.