Sorry for the delay -- Superbowl and all that stuff.
Labor Code 2802 requires that the employer must reimburse the employee for all expenses incurred within the scope of employment. In 2007, the Division of Labor Standdards Enforcement (DLSE) issued proposed regulations concerning employee travel expenses. These regulations were never finally adopted -- doubtless for political reasons, since it created a means of forcing employers to pay for mileage and motor vehicle operation for which employers have historically been able to avoid paying full value.
The regulations do provide an indication of how DLSE will rule, were a travel reimbersement issue brought to them for resolution. However, there is no guarantee that the Superior Court would rule similarly.
Similarly, in Gattuso v. Harte-Hanks Shoppers, Inc. (2007) 42 Cal.4th 554, the California Supreme Court held that: "Mileage rate used to calculate costs employee incurred using his or her automobile, for purpose of statute requiring employers to indemnify employees for costs necessarily incurred in discharge of their duties, may be set by agreement of party, but any agreement made by employee is null and void insofar as it waives employee's rights to full expense reimbursement. West's Ann.Cal.Labor Code §§ 2802, 2804."
In other words, the employer is required to reimburse all mileage which is incurred within the scope of employment.
Finally, in STEELE v. COMMISSIONER, 19 T.C.M. 966 (1960), the U.S. Tax Court ruled that only mileage between the place of employment and a temporary work site is deductible to the employee. In your scenario, the place of employment is your home office, because apparently your home is your office for the convenience of your employer. Therefore all of your mileage is reimburseable, under both California and federal law
Thus, the answer here is "yes," you can sue your employer for the additional miles -- assuming that you can prove your mileage with competent records. And, you can do so for miles that you can prove were driven back three years from the date that you file your lawsuit, pursuant to Cal. Code Civ P.roc. 338(a) (the statute of limitations).
There is just one "gotcha." If you were to simply sue in small claims court to recover your mileage, your employer can fire you for doing so. Believe it or not, there is no law that protects your right to sue over the employer's right to terminate you "at will" under Cal. Labor Code 2922.
However, if you were to file a wage claim with the DLSE, and DLSE accepts the claim and holds an administrative hearing, then your employer is prohibited from retaliating against you under Cal. Labor Code 1102.5. So, you may want to file a complaint first and see where it goes (click here).
I think that about covers the issue. Hope this helps.