You don't get to write off gas tax as an itemized deduction by itself. If you can claim your driving expenses as a write-off, though, you can deduct the entire price of the trip. There's no need to separate out the tax. Most driving-related write-offs are based on business use of your car, but there are exceptions.
If you make a deductible charitable donation, you can write off the cost of any related driving. As of 2013, that's 14 cents a mile, or the cost of gas and oil used for the trip. You can also take the deduction if you volunteer time or services to help the charity out. Sweat equity -- donating personal effort -- isn't deductible in itself, but the cost of traveling to volunteer is.
If you take the medical-expense write-off, you can deduct any medically necessary driving. You can take either the cost of gas and oil or write off 24 cents per mile. The deduction includes any trips you make for medical care, or the cost of driving your dependents to the doctor. It doesn't include dropping by just to cheer up a hospital patient, unless the patient is mentally ill and the visit is important for his mental health.
If you're an employee and your boss doesn't reimburse you for driving on the job, you can deduct the actual cost or claim 56.5 cents per mile. This is one of several "2 percent" deductions: lump them all together, subtract 2 percent of your adjusted gross income and write off the rest. Commuting to work is not deductible, but traveling from your office to a client or a job site is.
If you claim mileage, it's pretty simple: track how many deductible miles you travel along with the date of the trip and the reason. That's all the IRS needs. For actual expenses, calculate how many miles you drove this year, and what percentage was deductible. Claim that percentage of gas and oil expenses. For work expenses, you can deduct other maintenance costs. Parking fees and tolls are a write-off with either method. Keep records of your expenses for six years -- the IRS can't usually look any further back for an audit.
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