Standard mileage has its own component of depreciation.
Please see this answer from gloriah5200.
Yes, when you are selling or trading in a business vehicle, you must adjust your adjusted basis for depreciation allowed or allowable and if you used the standard mileage rate, you must adjust the basis for the allocation depreciation amount per mile for all business mileage deducted.
Depreciation component of standard mileage rate.
For computing the taxpayer’s basis in an automobile, the standard mileage rate has a component that represents depreciation.
For 2020, the depreciation component is 27¢ per mile.
If the actual expense method is used in any year after the standard mileage rate method has been used, the straight-line method of depreciation must be used. (Rev. Proc. 2004-64)
Depreciation Component of the Standard Mileage RateYear.....................2020..........2019..........2018..........2017..........2016
Rate per mile................27¢............26¢...........25¢............25¢............24¢
Note: These rates do not apply for any year in which the actual expenses method was used
