I’m a CPA (though taxes aren’t my specialty but I know enough). Yes you can deduct the difference as unreimbursed business expenses. However, you can claim it only if you itemize and you can only deduct the amount that exceeds 2% of your income. So if you made $25,000, you could deduct the difference in the mileage rate minus $500. However in 2018, this deduction goes away as part of the new income tax law.