When you write a check to your state or county for your car registration and tax, you might not pay much attention to the breakdown of the total. However, the Internal Revenue Service does, because you're only allowed to write off the portion that's based on the value of your car. If your state includes a flat registration fee as part of your registration, that part isn't deductible.
Claiming the Deduction
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On your tax return, you can write off the property taxes on your car as a personal property tax. You report this deduction on Schedule A, which means you can only claim the deduction if you're itemizing. Unless you have an incredibly expensive car, chances are you're going to need some other itemized deductions, including charitable deductions or mortgage interest, to make it worth giving up your standard deduction.