Tax Year Prior to 2020: How do I dispose of a vehicle used for 1/2 the year, and then traded in for a new vehicle used for the other 1/2?

Is this a personal vehicle or business vehicle?

I have been deducting the business mileage each year (which varies) and it amounts to approx. 10% of the total, with the remainder being personal use.  As it appears, I must now use that mileage amount as a deduction against the trade-in value of the vehicle for a new vehicle in which I will do the same, roughly 10% of the annual miles for business use.

The best way is to deduct the business mileage for both vehicles. It doesn't really matter whether your vehicle was traded in or not. You are eligible to deduct the business mileage for both vehicles. Ensure you have maintained a log of your business usage for both vehicles.