You can use either method as long as it's consistent. You need to track your business and personal use miles in both cases and enter them to a tax software when you calculate your vehicle expenses, If you don't provide your mileage, no deductions will be allowed for your vehicle.
If you want to use the standard mileage rate to calculate vehicle expenses, you must choose it in the first year you use the car for business. In later years you can choose to use the standard mileage rate or switch to actual expenses.
Once you use actual expenses for the vehicle (even if it's the first year you used it for business), you can't switch to standard mileage rate. You must continue using actual expenses as long as you use that car for business.
