You should make an attempt to create a reasonable estimate of your business miles. for example, if you drove the same route multiple times you could estimate the approximate number of times and the mileage. You are really required to keep written records of your mileage for business, so do the best job you can to estimate that mileage, put it in writing and describe how you arrived at the numbers. In an audit, the deduction can be disallowed without good written records.