Ah yes now I understand. If you never used a car for business and sell it for a loss, it's tuff luck and there is no tax deduction. If you sell it for a gain then you pay taxes. Kind of a double standard but that's the deal. If the loss was purely due to business use, then you could take a loss on the 1040 return.
I just went thru this on the sale of a partial business use car. I had to dig up 19 years of returns. I had used the standard mileage rate for all 19 years. I used a table the IRS provided for the specific depreciation allowance for each year (Cents per mile for depreciation) times the business miles each year. This is not the same table as the total mileage expense allowed for each year, it's just for the depreciation portion. Then I calculated overall business use percentage (total Business miles/Total miles) to figure the business cost basis and business sale price. Then I used the total depreciation over 19 years to determine the business adjusted cost basis.
