There is nothing to claim for the purchase of a new car, unless you are itemizing your deductions and are deducting sales tax. You could also have ad valorem tax to deduct. Regardless, if there's something to file or claim it's yours, unless your grandfather's name is also on the title and not just the loan. If he's not a co-owner, the car and any possible tax deductions are all yours. He's just on the hook if you don't pay the loan.