The Internal Revenue Service (IRS) requires you to calculate your deduction in one of two ways, depending on how the charity uses your donation. Deductions for cars the charity sells are limited to the sales price. In all other cases, you can use the car's fair market value.

There are the four specific conditions in which the IRS allows you to claim the maximum deduction (fair market value) of a donated car: When the charity auctions the car for $500 or less, you can claim the lesser of the fair market value or $500.

If the donated vehicle is sold for less than $500, you can claim the fair market value of your vehicle up to $500 or the amount it is sold for if less than fair market value. If the donated vehicle sells for more than $500, you can claim the exact amount for which the vehicle is sold.