I think what you mean to say is something to the effect of "We got an interest-free loan to buy the car" or "Our car loan is interest-free". In that case, if you wanted to say that your interest rate was 15% instead of 0% you might say something like "We're paying 15% interest on our car loan" or "We could only get a loan at 15%" or "Our car loan is financed at 15% interest".