You can calculate your vehicle deduction using the standard mileage method or the actual expense method. The standard mileage method is more commonly used because the record-keeping requirements are much simpler. Under this method, the IRS determines the amount you can deduct per mile. The rate for 2012 is 55.5 cents per business mile driven, which is the same rate that was in effect for the second half of 2011.
Under the actual expense method, you deduct the actual costs you incur each year to operate your car, plus depreciation you pay for gas and repairs (according to a tax code schedule). Your deductible costs include gas and oil, repairs and maintenance, license fees, insurance, tolls, and even car washing. If you use the car partly for personal use, you must multiply your actual expenses by your percentage of business use.
Most people use the standard mileage rate because they don't want to bother with a lot of record keeping. But this ease comes at a price -- you usually get a lower deduction using the standard mileage rate than you would with the actual expense method. You must use the standard mileage rate, however, if you claimed certain related deductions (such as under Section 179 of the IRC) in previous years. (For more information on Section 179 depreciation, see Nolo's article Small Business Tax Deductions.)
To use either of these methods, you must keep track of how much you use your car for business. (And you'll need to produce your records if you are audited.) Keep a log showing the miles for each business use, always noting the purpose of trip.
You can also depreciate (write off) the cost of the vehicle over a number of years. For more information, see Deduct It! Lower Your Small Business Taxes, by Stephen Fishman (Nolo).