How to calculate business mileage deduction when mixing personal travel - driving from NorCal to SoCal with side trip

You can only deduct business miles, not personal miles. In your situation, there are two correct approaches depending on how you look at it: Option 1: You can deduct the actual business miles driven, which would be just the Albuquerque to San Diego portion since that's the only segment being driven for business purposes. Option 2: You can deduct the "normal" business mileage that would have occurred without the personal detour, which would be Sacramento to San Diego. The IRS considers this reasonable since you're not trying to deduct any extra mileage caused by the personal trip. What you CANNOT do is deduct the entire trip (Sacramento → Albuquerque → San Diego), as the Sacramento to Albuquerque portion is clearly personal. I recommend documenting your usual business mileage pattern to support whichever method you choose. Keep a mileage log with dates, business purpose, starting/ending points, and odometer readings.

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A tax tip most people miss: if you're going to make a mixed trip like this, consider structuring your travels to make more of the mileage qualify as business. For example, if you're already in Albuquerque, see if there are any potential clients, suppliers, or business opportunities you could pursue there before heading to San Diego. If you can document legitimate business purposes for contacts in New Mexico, you might be able to convert what would have been purely personal mileage into partially business mileage. Just make sure you document everything thoroughly - names, dates, business purpose, etc.

I'm confused about something - if I'm on a pure business trip but I stop for dinner or to see a tourist attraction, does that somehow disqualify those miles? Like if I drive from Sacramento to San Diego for business, but take a 30-mile detour to see something cool, how do I calculate that?