Actually the IRS is the best place to get answers to tax questions. You can depreciate an old car IF you just bought it and put it into service the first year you bought it. Otherwise you must use standard mileage deduction. They consider it 5 years depreciation, which means your car was fully depreciated before you put it into use.
My 5 year old car was just purchased, and put to use immediately. Therefore I take the purchase base with taxes and registration, and multiply by percentage used for business. That number is then divided over 6 years, 1/2 a year the first year, half a year the 6th year. If you qualify for both deductions, see which type gives you the better return.
