If you are using the standard mileage method, the key number is the number of business miles driven. You should have a paper or electronic log showing the date, purpose, and mileage of each business trip. The total vehicle miles for the year don't really matter, so if the program asks, you can make your best guess.
If you are using the actual expense method, you must know the total number of business miles driven AND the total number of vehicle miles (beginning and ending odometer) so the program can calculate the percentage of business miles. In addition to the log mentioned above, you need records of all your repair, maintenance and fuel expenses. If you are audited, and can't adequately prove your expenses, total miles, or percentage of business miles, the IRS could reduce your deduction.
