Depreciation equivalent for total miles is a small portion of depreciation that is included (by default) when using standard mileage rate for your business car. When you take standard mileage rate while deducting your business car miles, you must account for 'depreciation equivalent' when disposing of that car.
The IRS provides a table with the depreciation equivalent rate for each year (page 24 of the link, IRS publication 463)
2012 and 2013: rate is $0.23/mile
2014: rate is $0.22/mile
2015 and 2016: rate is $0.24/mile
Example: You drove 1000 miles as business miles in 2014, 2015 and 2016 and took standard mileage deduction for each year. To calculate your depreciation equivalent - when disposing your car - you must know how many business miles you drove and the depreciation equivalent rate from the IRS (which I listed above). The calculation is as follows:
2014: 1000 miles times $0.22 rate = $220
2015 and 2016: 2000 miles times $0.24 rate =$480
Your total depreciation equivalent (also called total prior depreciation) is $700 (the same amount applies to AMT). Note: When depreciation equivalent is added, it changes your cost basis for the car.