if you used the standard mileage method, TurboTax does not include depreciation in calculating gain/ loss. you have to calculate it using each year's mileage rate found in Pub 463 page 35
https://www.irs.gov/pub/irs-pdf/p463.pdf
also if there was personal mileage, you can only take a loss on the business portion, which is figured by multiplying its cost by the business mileage all years divided by total mileage all years. the trade-in value would also be prorated the same way.
for the personal portion, if any, gain is taxable, while any loss is not deductible.
