it's complicated if business use wasn't 100%. there is no longer like-kind/ trade-in treatment for vehicles. you are treated as selling the old vehicle for the trade-in value and purchasing the new one for the cost excluding the trade-in value (assuming these are realistic - dealers can manipulate the numbers any way they want so the cash they get is the same. then for the trade-in, the proceeds need to be allocated between business and personal use. the most logical way I can come up with is to allocate based on business mileage to total mileage. since the personal portion doesn't depreciate for tax purposes any loss on the sale of that portion isn't deductible. any gain, unlikely is taxable. then there are the proceeds allocated to the business portion, the business portion of the cost (again most logical would be to allocate on mileage) and the depreciation taken which is complicated if you used the standard mileage method because a portion of that is depreciation. the depreciation amount per business mile is in IRS PUBL 463
https://www.irs.gov/pub/irs-prior/p463--2019.pdf
for 2020 the rate is 27 cents.
so what you need to do in years you used the standard mileage rate is to compute depreciation by multiplying the rate for that year by the business mileage. if used actual costs for some years the total depreciation is the sum of what was deducted under each method. it can not exceed the business portion of the vehicle.
you gain or loss is trade-in value less the net of the business portion of the cost less depreciation
