There is no reason to track your mileage for federal tax purposes, since you cannot claim a deduction for the mileage. The car allowance is just additional income on your tax return. You don't have to explain it.
There are two reasons that you might still have to track your mileage.
1. Your employer might require you to report your mileage to the employer in order to get the car allowance.
2. Some states still allow a deduction for job-related expenses, even though the federal tax law doesn't allow it. If you are able to itemize deductions on your state tax return, and your state allows the deduction for job-related expenses, you would need to track your mileage to calculate and support the deduction on your state return. This only applies if the car allowance is reported as taxable income. If the car allowance is paid through an "accountable plan" and is not included in your taxable income, then you cannot claim a deduction for mileage.
If neither of these two situations applies, there is no need for you to track your mileage.
