In 2018: How do I figure the depreciation total for the sale of a business auto, used standard mileage rate every year for 10 years (business use under 50% every yr)?

Do I total all depreciation dollar amounts claimed each year, or is there a set amount allowed per mile for each year for depreciation purposes when figuring the sale? Say the car cost $20,000 in 2008, & total business miles are 5000 through 2018 (30% business use). Thank you!