Why a car loan APR is higher than a Mortgage APR?

ok this is my situation I got a mortgage back in October 2010 with a credit score of 535 and my wife Score of 530, the APR that the bank gave us is 4.9 for 30 years (bank of America). I have two cars loans with Honda Financial just once late about 3 years ago. I went to get a car a few days ago the scores still low in 530s and the dealer told us that the APR will be about 18%, they did not send to the banks they just looked and they told me because my score most likely that's the APR I would get. I still have just two small accounts in collection that i am paying, also we have no repossessions we make about 82000 a year and one car is already pay off. Can anyone explain me this why the APR on car loan is way higher than a mortgage? Is there anyway that I could get a better APR on my car that i am buying?