Deductions & credits
If you fully depreciated the vehicle, your gain will be the sales amount less costs of sale.
When you go through the sales routine in TurboTax it should have asked you for the depreciation equivalence amount based on the miles you deducted during the period of time you used the vehicle for business purposes. You have to factor the depreciation into the sale for gain or loss purposes.
The cost basis of the vehicle will be the original cost less depreciation. Here is a table of the depreciation cost per mile, just multiply your business miles by the factor for the applicable years and you will have the depreciation for that year:
