How could I deal with an audit regarding mileage vs actual vehicle expenses?

I believe that the revenue agent performing your audit is confused. She is classifying your use of the car as "for hire" because your compensation is based on miles traveled. However, the IRS definition of "for hire" use is used in the transportation of goods or people. It is often pointless to attempt to persuade a revenue agent, the ones assigned to individual audits aren't accountants, they have little understanding of the law and their training is formulaic. To them every issue is black and white, once they have decided how they will handle an item, you can never change their mind. I would advise you to assemble the information she requested. You might be surprised and the actual expense method might give you a bigger deduction. If the actual expense method doesn't, when you get the audit report (the so called 30 day letter) request a determination by the irs appeals office. These people are usually accountants and lawyers and they understand the complexity of tax law and how to use references. Since you do not transport goods or people, they should agree you qualify for the standard mileage method. Since this determination will be based on interpretation of IRS publications and tax cases, you should probably be represented.

See a tax attorney or accountant to review the IRS allegations and assist in the documentation of actual expenses if required.