This is a suggestion regarding the expense management module however there doesn't seem to be any category out there for expense management or travel and expense.

The mileage reimbursement rates can change mid year (as they are this year), D365 should have the capability of putting in effective mileage reimbursement rates by a date range. Currently this does not exist. Everything is drive off of the the setting in the Expense Management Parameters and mileage tiers (which has date ranges, but are overridden by whatever is set up in expense management parameters which only allows for a single reimbursement rate to be entered.