"Why, it's zero percent!" would be an appropriate answer to the question "What percentage rate are you paying on your car loan?" Free of the constraints imposed by the structure of that question, however, you could surely find a better way of expressing that idea than "zero-percent financing." "No-interest financing" works, for instance, as does "interest-free financing." Or you could simply call it "free financing," just as you'd refer to something as "free" and not "costing zero dollars and zero cents."