My question involves labor and employment law for the state of: California

My spouses employer pays a flat rate per day ($10) for a car reimbursement in lieu of mileage. Is this even legal? Our personal car is used to drive approximately 200 miles per week on top of the 200 miles that are driven as commute to and from the home office. As part of his job he has to commute to many different locations. Thank you!