How do I calculate the depreciation equivalent on my car mileage? Do I add all of the years together?

[JohnW15]

Not quite -- as you might expect, it's not that simple.

The depreciation equivalent for total miles is small portion of depreciation that's included when using the standard mileage rate for the business use of your car.  When you claim the standard mileage rate when deducting your business car miles, you have to account for 'depreciation equivalent' when you dispose of the car. 

The IRS provides a table with the depreciation equivalent rate under "Rate of Depreciation Allowed in Standard Mileage Rate" on page 23 of the 2018  Publication 463, Travel, Gift, and Car Expenses.

If you'll see the example on page 24 of the publication above, you'll see how the depreciation equivalent is figured.