auto loan rates

In reading, many people recommend having either one or two revolving credit lines, and one or two fixed credit lines, for a total of three lines of credit. My credit card is one revolving credit line. I’m curious if taking out say a four year auto loan and making payments on time would substantially improve my credit score three years from now. The loan amount would likely be $7,000 to $8,000, giving monthly payments of around or under $200, which I can realistically afford. I’ve been paying my own college tuition and rent since the second semester of my freshman year of college, so I somewhat have of a grasp of my expenses.

On a side-note, I’m under the impression that I would be stuck with the interest rate I initially receive. I could refinance, but wouldn’t that close out the line of credit and begin a new line of credit, potentially lowering my score?

The follow-up questions are, would it be in my best interest to open a second credit card for additional feedback on my credit history? Also, I currently have a good amount of student loans, all of which are in deferment until I graduate (if I attend grad school, they will remain in deferment through that period). Do these loans count towards my credit score as fixed lines of credit? Some are federally-funded, while others are through private institutions.

For what it’s worth, Credit Karma reported my credit score at 722 as of a week ago, and TransUnion reported it at 764.

Delores