You can estimate
the total mileage because you will need it in order to claim the business
mileage.
Be aware though that in the event that your return is audited
for some reason and the mileage is examined, the IRS will expect some
documentation justifying the business miles and the total miles. Check you oil change or auto repair records
near the beginning and the end of the year.
They generally would include your odometer readings.
