Can my employer make me use my own vehicle for work?
But it's probably not the best idea that they do that, from their perspective, because it opens them up to liability in situations they might not otherwise have it. From yours, it means that you incur expenses in gas, wear and tear, and so forth. You're entitled to reimbursement for those expenses under Labor Code section 2802. There are handy charts that can tell you how many cents per mile you should get.
UPDATE: The DLSE considers the rates set by the IRS to be sufficient:
DLSE has opined that use of the IR S mileage allowance will satisfy the expense s incurred in use of an employee’s car in the absen ce of evidence to the contrary.
Disclaimer: The above advice is not intended to reflect any actual situation and reflects a casual scan of the law as understood by the author at this moment. Anyone who needs legal advice to meet their particular situation should retain counsel, and SIOULD NOT rely on the above.