The Internal Revenue Service allows you to claim a tax deduction for business mileage. The IRS defines business mileage as miles driven for your job or business. If you do not use your car exclusively for your job or business, the IRS requires you to prorate your usage of the vehicle into personal and business miles. You can manually calculate the personal mileage from the business mileage figure for any tax year.

Determine the total mileage for the tax year. Subtract your ending mileage on December 31 from your beginning mileage on January 1 of the same year. For example, assume your mileage on January 1 was 35,000 and your mileage on December 31 is 60,000: 60,000 - 35,000 = 25,000.

Determine your business mileage for the tax year. The IRS recommends keeping a daily log and recording the actual miles driven for your job or business. For example, assume during the year, you drove 10,000 miles for your job.

Subtract your business mileage for the year from your total mileage per year. Continuing the same example, 25,000 - 10,000 = 15,000. This figure represents your personal mileage for the year.