TurboTax asks for both the total mileage as well as the business mileage in order to calculate the % that a car was used for business purposes using the 'Actual Cost' method.
If the business percentage of your actual cost method produces a deduction that is lower than the standard method, then the standard method will be used instead (business mileage only).
IRS Guidance: https://www.irs.gov/uac/2017-standard-mileage-rates-for-business-and-medical-and-moving-announced
Helpful Examples: https://turbotax.intuit.com/tax-tools/tax-tips/Self-Employment-Taxes/Standard-Mileage-vs--Actual-Exp...
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