Keeping track of mileage is as simple as creating a new expense for it. You can also create a custom expense category for it:
1) Open a new expense.
2) In the expense, scroll down and click on the "Category" field.
3) Click "Manage Categories".
4) Click "Add New Category" to add "Mileage" and save it as a new category.
4. Now you'll have the "Mileage" category in your "Category" drop-down list.
What Happens Now?
Now you can go to your Expenses page, click on the 3 dots menu on the right hand side of the "Expense" column header, and then hover over "Filter".
From there you'll see two filter. You'll want to switch both of them to "Contains", and then type in "Mileage" in both filter, and then click "Filter".
Now you'll only see all your mileage expenses.