If you call and ask the California Department of Labor, no one needs to lose their mind. You just ask, and the CADL will be glad to answer your question. Your employer need never know you called them. You explain the situation to them. They will tell you the answer. If you have the law on your side, then you certainly can speak to your employer with more authority. I honestly do not think that you do, as I have seen a lot of travel pay outs, and usually, in what I've seen, you were paid for mileage from your base workstation, rarely from your home, unless this was somehow negotiated at the time you started working at this job.
It would be possible to have a forty mile commute from your home to the worksite, and someone else who works there have a three mile commute to the workstation. If both of these folks drive over to the other worksite in another town, should one be paid for forty miles more simply because they elect to live forty more miles from the central workstation? What if where you are going is closer to your home than your regular worksite, do you get paid less if you drive there straight from your home?
Why not ask the folks who know all the law and ins and outs, and then, armed with information straight from the horse's mouth, you can take this up with the boss, knowing that if they cut a shine, and it comes down to asking the CA Wage and Hour to get involved, you'll be more likely to win reimbursement for travel. Even though you may be fired by this time, you know.

