Even if you are using the standard mileage rate you MUST enter in the total miles used for the entire year, the business miles and the personal miles. The deduction will be based on the business miles... it is not reduced by the personal ... not sure where you think that is happening.
If you use the actual expenses option then you must also enter the mileage but also ALL the expenses for the year on that vehicle so the total of the expenses can be prorated between bus use and pers use correctly.