Even though Outlook has a field to record mileage for various items, aside from the Tasks item, it is nearly impossible to fill it out quickly.
While you could customize your Appointment and Contact forms or create a Form Region to make the field more readily available, those solutions are a bit of overkill just for this field alone. Another way to enter it would be to use a list view and enable in-cell editing but that would be quite cumbersome when you need to do it on a regular basis.
This guide contains a code sample which you can use to quickly log and retrieve your mileage for a selected or opened Appointment/Meeting, Contact item or Task.
Continue reading: Record gas mileage for Appointments, Meetings, Contacts and Tasks