Question regarding fuel and mileage write off

Keep track of where she goes for work and how many miles she drives. At the end of the year, you can write off whatever the IRS sets the mileage rate for that year (this year is $.555/mile, I believe). Also keep track of trips to buy office supplies, trips to the post office, etc., as those trips are also deductible.

You can also write the proportion of interest you used your car for business (ex: if you used your car for 75% business and paid $1000 in interest on the car loan, you can write off $750)

I keep a yearly record of beginning mileage, ending mileage and the miles that I drove for business in an excel spreadsheet.

First off, you have to be able to itemize your deductions. If you take a standard deduction, then there is no place to write off mileage. If you itemize, then the mileage will be deducted on your 2106 Business Expenses. Once you figure all of your wife's expenses, then that total will be reduced by 2% of your Adusted Gross Income. That amount will be the write off.