Chris,
I was a dealer/consumer loan officer with a bank for 18 years in another life. Your interest is calculated daily and paid monthly with each payment. The first dollars of each payment go to pay off whatever amount the accumulated interest is. The rest of the payment goes to principle after the interest is paid. This is true of all simple interest installment loans. So, any extra money you pay on each payment goes directly to principle thus effectively paying the loan off early. This should also pay the loan due date out into the future, example: If you pay an extra amount equivalent to a full payment, not only will the extra pay off the principal faster, it will make the next payment due date one month out into the future. I do this all the time on my vehicle and home loans. It gives me an extra cushion if some extra expense happens in life, I don't have to make that payment if I don't want to. Hope this helps.