The best way to do this is to pay for the entire car either, including gas, insurance, and repairs, from S-corp funds, then meticulously track how many miles are used for personal and how many miles for business.
If you pay with S-corp funds, you will claim the personal miles as a taxable benefit from the S-corp on your personal return. The S-corp can then claim all the expenses and depreciation on the vehicle, reducing the S-corp's tax liability.