>At a 5.5% interest rate - why is 15% of my monthly payment going to interest? Why shouldn't it be... 5.5%?

Because you're not paying down the loan in a single year.

Consider the following cases:

• You pay the loan in full the day after you buy the car. You would pay effectively no interest regardless of your rate, because you didn't keep the loan very long (there might be some minimum duration or something). Your interest would be almost none of the amount you pay, or
• You never pay off the balance of the loan, paying "interest only" and keeping the loan balance the same (this is common in business loans). 100% of your payments would be interest, again regardless of the rate.

When you pay off the loan in a fixed time period, it's somewhere between these two extremes. The interest rate is the percentage of the balance that is charged as interest - if the outstanding principal is $10,000 and your rate is 5.5% then $550 of interest will be charged on an annual basis. The amount of principal you pay in addition the interest is calculated in an amortization schedule in order to both:

• Keep your payments the same amount every month, even though the amount of interest accrued each month will shrink as the balance goes down (you'll note that the interest portion of your payment shrinks each month), and
• Pay off the loan in a certain number of months