If box 4a on the Form 1098-C is checked, then you must use the value in box 4c of the 1098-C when you enter your vehicle donation in your tax return, except in two very unlikely situations. Those unlikely situations are that the fair market value of the vehicle on the date of the donation was less than the amount in box 4c, or that the fair market value of the vehicle on the date of the donation was more than you paid for the vehicle when you bought it. In most cases you will just use the amount from box 4c as the amount of the contribution.