Keep a log book in your vehicle throughout the year. Mileage for taxes is tracked from January 1 through December 31, and unless you use your car exclusively for business, you should write down the beginning mileage, the ending mileage and the purpose of the trip each time you use of your car for business.
Subtract the beginning mileage from the ending mileage for each trip to determine the miles driven for that trip. Do this on a regular schedule, such as once a week or once a month, to maintain the ease of tracking total mileage used.
Add the total miles driven after the end of the year to determine your total mileage for taxes. Multiply this number by the allowable deduction for vehicle use.
Enter this amount in the mileage deduction line on the tax form you are using, such as Schedule C for businesses (or when asked for the amount on an online tax program).