From Car Loan Wiki

The interest rate index is the rate determined by the supply and demand for credit in our economy. The more demand there is for credit, the higher the interest rate index percentage will be. For example, when you deposit money into a bank account, you are allowing the bank to use your funds to lend to someone. This act is actually increasing the supply of credit, therefor lowering interest rates. On the other hand, if you don't pay off the balance of your credit card, or you take out a loan, you would be increasing the demand for credit and ultimately raising interest rates. So if you get an auto loan with a variable APR and the interest rate index goes down due to a decrease in demand for credit in the economy, your auto loan's interest rate should decrease accordingly. The interest rate index can also be affected by inflation. When inflation is rapid, you can generally expect interest rates to go up. Watching and learning how the interest rate index works, and keeping an eye on trends in the economy, specifically the supply and demand of credit, can help you save a lot of money on variable rate auto loans.

To see current interest rates, check out the Auto section of Bankrate.com