http://turbotax.intuit.com/tax-tools/ta … 19464.html
Accountable plan reimbursements
If your employer has a policy that covers reimbursements or allowances for mileage, you need to determine whether the policy is an accountable plan before claiming any deduction. Assuming that all mileage covered under the policy solely relates to your employer’s business, the IRS treats the policy as an accountable plan if you must account to your employer for your business automobile expenses and return any excess reimbursement within a reasonable period of time.
If these conditions are met, your employer doesn’t have to report the reimbursements as taxable wages on your W-2, which means you don’t pay income tax on them. But, since you receive tax-free mileage reimbursements, it means you’re precluded from also taking a deduction for the same mileage expenses. However, if your reimbursement or allowance doesn’t cover the entire expense, you can deduct the unreimbursed portion as if no reimbursement policy exists. Your Form 2106 will show your expenses and the amount of employer reimbursement; the difference between the two will be your deduction.
Other reimbursement policies
Your employer may reimburse you for using your car at work, but, if the payments aren’t made pursuant to an accountable plan, your employer has to include them on your W-2. As a result, the reimbursements will be reported on your tax return in the same way as your wages. Although you will pay income tax on your reimbursements, you can deduct all mileage expenses despite receiving reimbursements.