Some lenders do this automatically, some do it if you request it, and some make it extremely difficult. Before assuming that any extra payment goes to principal rather than just future payments (and you DEFINITELY want it going to principal!), take a good close look at your payment documentation ...coupon, webform, whatever.. and make sure it states where extra payment goes, or that there's a box for you to indicate that any additional payment should go to principal. Mortgage holders are generally pretty good about this. Auto loan corportations, especially Toyota Finance Corp, are so bad about this it borders on criminality. A few years ago Toyota offered me a 0.9% loan on a car which I accepted rather than use money that was earning much more than that. I specifically asked the loan officer if I could send additional to principal monthly and he replied that yes, any additional money would to go principal. 5 months later (my wife does the actual check-writing/physical bill paying), my wife shows me a statement from Toyota saying we didn't have to send in a payment because we were paid up for the entire year...they had been applying the payments to future payments, including future interest we hadn't even incurred yet. I called them, and was told that if we wanted to pay additional to principal, we needed to send a SECOND different check each month to a different address, and the only way to find that out was to call them. I was incredulous, and mad them repeat that. They confirmed it...that procedure is not listed anywhere, they were aware of that, and confirmed that it was intentional on their part. I paid off the entire loan immediately.
Soooo... before ASSUMING additional payments go to principal, ENSURE that they do.