This question concerns employers requiring employees to use their personal vehicles for company business, in this case a job entailing driving my vehicle off-road either to get to the work site or on the work site.
My questions are:
-Can my employer require me to use my personal vehicle in these conditions?
-Are there any stipulations to expense/mileage reimbursement law that allow for extra reimbursement for the more than "normal wear-&-tear" that my car is undergoing?

I work for a private (California) environmental consulting firm whose contracts involve a fair number of projects that are located in remote/rugged locations and/or involve off-road driving within the project area (anything from roadless areas to large construction sites); a truck &/or 4WD is often neccessary. I use my personal 4WD truck to either get to the job, access the work area, haul & store company equipment to and from the work site, transport crew, or all of the above. I'm allowed to claim mileage reimbursement at a normal ($.375/mi.) rate for miles driven at the project area. There is no company vehicle for use.

My issue is not to try to rack up extra reimbursement, but the real concern I have for the condition of my only vehicle and the extra maintenance. I would prefer not to use my 19 yr-old vehicle but my employer hasn't and is probably not going to make effort to purchase a company work truck anytime soon.

Thanks for any advice/input.

Fair Labor Standards Act indicates that the employer doesn't have to reimburse you at all for mileage/gas unless it causes you to drop below the minimum wage. I don't see anything about it for california (which doesn't mean there isn't something...but I don't see). You can contact the Department of Labor wages & hours division in california for more info.