Legal Question in Business Law in California

I don't know if "force" is the right word; your work relationship may be employer-employee or independent contractor, but it isn't slavery, and you have a voluntary agreement of some sort in place with the employer. In general, the terms of such voluntary agreements are left to the parties, not dictated by law. There are, to be sure, major exceptions for things such as safety, minimum wage, etc. etc. -- but items such as reimbursement for auto expenses aren't controlled by statute, they're left to the parties to work out as part of their agreement terms. Theoretically, the employer could require you to furnish your own gas at your own expense. Sure, they'd have to make up for it with higher base pay or something, or they'd have a hard time attracting and keeping employees or independent contractors -- but it'd be perfectly legal.

Mr. Whipple is wrong. There is a specific statute labor code section 2802(a) that requires an employer to reimburse employees for their expenses. There is a whole raft of regulations that define what is and is not a reimbursable expense. When it comes to auto expenses, however, the rule is simple. If they are paying you the IRS mileage rate for all miles driven, they are legally deemed to be reimbursing you enough, and yes, they only need to reimburse you monthly. If, however, they are only paying for gas, then they are not fully reimbursing you and you have a legal right to additional reimbursement for your actual per-mile costs. But here's the problem, you will have had to document all your expenses and your miles driven for both work and personal use, or at a minimum have kept mileage records and go with the difference between the IRS rate and your gas reimbursements.