You can deduct your unreimbursed business mileage expenses or your actual business vehicle expenses, whichever is greater. You would need to keep a mileage log of how many miles you drive during the year total, along with business vs. personal. You can then determine your business use percentage and multiply your actual expenses against this. This will give you your actual vehicle expense deduction amount. Then you can multiply the amount of business miles by the current year standard mileage rate. If this amount is greater than the actual, you can go ahead and use it instead. But you can only use one or the other. For example:
Let's say you drive a total of 20,000 miles for the year, 12,000 are for business. Your business use percentage is 60% (12,000/20,000). If your total actual expenses for the year including gas, repairs, etc. are 5,000 your actual expense deduction is $3,000 (5,000 X 60%). While your standard mileage deduction for 2010 would be $6,000 (12,000 X .50/mile). Therefore, you would be able to go ahead and take the greater deduction which would be the $6,000 standard mileage deduction.