I keep 2 vehicles, my jeep and my Range Rover, I write off ALL fuel, service, tires, mileage, etc for the Rover which I dedicate as my "work" vehicle. This makes it easier to total out the deduction at year end.
If you classify it as mixed use, break down the mileage and pro rate the portion allocated to business use (deducting commuting mileage from the total). PM me if you want a more detailed breakdown. I'd be happy to help. Saves me a couple thousand a year