What kind of mileage log does the IRS actually accept if I get audited?

The IRS wants a contemporaneous record, meaning one kept at or near the time of each trip, not reconstructed months later. An acceptable log shows the date, business purpose, starting and ending points or destination, and miles driven, plus your total annual mileage so business use can be expressed as a percentage. A mileage-tracking app that logs trips automatically via GPS is ideal and time-stamped. A paper notebook in the glovebox also works if kept faithfully. Odometer readings at the start and end of the year strengthen it. What fails audit is a single spreadsheet made in April with suspiciously round numbers. Consistency and detail are what make the deduction stick.