I am reimbursed for mileage at 1/2 of the IRS rate. Can I write off the other 1/2 of my mileage?

My company provides me with a monthly allowance, that is taxed, for me to purchase/maintain a car to use for business. They also reimburse my mileage at 1/2 of the IRS mileage reimbursement rate. Can I write off the other 1/2 of the mileage reimbursement rate? If so, which Turbotax product is best to use for this situation?