Deductions & credits

If you are self-employed, the standard mileage would be listed on Schedule C, part IV (part 4).

If you are an employee - the mileage would be listed on Form 2106, Part II (part 2).

2017 Instructions for Schedule C (2017) https://www.irs.gov/instructions/i1040sc
Form 2106, Employee Business Expenses  https://www.irs.gov/pub/irs-pdf/f2106.pdf

Can I switch between the standard mileage rate and actual expense method? https://ttlc.intuit.com/replies/3302032