How to calculate how paying extra principle on a loan changes payoff date?

This is about my car loan. I know that a portion of each payment goes toward interest and the remainder goes toward the principle.

I want to pay an additional $200 per month, but how do I figure out how much that will decrease the length of the loan? Do I just knock an additional $200 off the principle month by month until I figure it out (this sounds tedious, is there a program online that will help? Similar to an amortization schedule.) or do I need to somehow account for interest as well?

Me + math & finances = confusion