A mileage log is a written record of your trips that you submit with your business mileage deductions. It should show the date, destination, business reason for the trip, odometer readings, and total mileage. If you drive your vehicle for both personal and business use, you can only deduct the mileage that qualifies as business-related.
The IRS requires that you keep your mileage logs for three years—whether a paper-and-pencil mileage log you keep in your glove box or a digital mileage log stored on your phone.
As the taxpayer, it’s your responsibility to document all business mileage claimed, and to report it accurately when filing taxes. If you’re audited, the IRS will look for documentation to substantiate that mileage was in fact business-related. Without such documentation in the event of an audit, the IRS could disallow your deductions, costing you far more than the hassle of jotting down notes about your trip.