Firstly, navigate to your Company Settings.
Next click on the More tab and then Expenses.
Scroll down to Expense types to view a list of existing types. Here you can add a new type, or edit or delete an existing type.
Adding a New Mileage Type
Clicking on Add new expense type will generate a dialog where you can enter the details for the expense.
Here the Expense type can be either General or Travel. Selecting Travel will show the following warning notification
This means that any expense type that should have a mileage allowance attached to it should be set up in the Mileage allowance section rather than here.