What documentation is needed to deduct auto expenses?

You should maintain a log or diary of your auto expenses. If you use the actual expense method you should save the actual receipts for gas, oil, insurance, repairs, lices and taxes. If you use the mileage deduction method you need to maintain a log or diary which lists the date, destination, business purpose, mileage driven (odometer start/end) type and amount of other expenses incurred. Currently there are phone applications for mileage documentation.