Keep in mind that average age of credit lines, percent of total available credit used, and variety effect your score. It's not meant to be punitive, and it's not perfect, but assess risk in a variety of ways. Paying off old credit lines that close (student loans and car loans for example) are more damaging to you the fewer other lines you have.

The best way to protect your score for impact is to have a few credit cards, you never close, and you keep working towards increasing the limit on.

This will drive your credit age up over time, give you a larger pool of available credit the system is working with, and mean that a single car loan being paid off will have a smaller impact on the amount of credit lines you have.