Here's the true scoop:
Your auto loan works like this:
Loan balance times interest rate times # of days since last payment divided by 365 = interest deducted from your payment. The balance of your payment is applied to the principal balance.
IF you want to pay your loan off faster by making extra payments, DO NOT send in a separate payment. The financial institution will simply apply the above formula, assuming you are making the next payment early. Here's an example:
You make your normal $500 payment that's due on 10/15/03. One week later, you decide you want to make an additional payment on your loan and send in a payment of $1,000. The financial institution will simply assume that you are making the 11/15/03 payment early, calculate the interest since the last payment (seven days interest) and apply the remainder to the principal balance of the loan. They DON'T care what your instructions are, because they don't know for sure that you will be making the 11/15/03 payment - so, they make sure they get their interest income first - before anything get's applied to your loan balance.
IF you want to be sure that the additional amount gets applied fully to your loan balance, your 10/15/03 payment should be for $1,500 - that way the extra $1,000 is assured of getting applied to you loan balance.
Any other manner of loan accounting is illegal - the financial institution can only charge you for money that you have borrowed.