The deduction permitted by the IRS for the expense of operating a motor vehicle. A taxpayer can choose to use the standard mileage rate as opposed to maintaining a detailed record of expenses that are actually incurred. The IRS identifies three standard mileage rates depending on whether the vehicle is used for business purposes, charitable causes, or for medical or moving purposes. The standard mileage rate is based on an annual study of the fixed and variable costs of operating a motor vehicle.