As Slyvia has said it is a judgment call on your part on whether or not you want to pay off your auto loan or not.
IMHO I feel the less debt and obligations to creditor's trump the FICO score everytime. For one, you will now have more money in your pocket that you haven't had for years, that you can use what ever way like ( start a vacation fund, increase your savings, get caught up on other debt etc..)
As it does seem when paying off an installment loan it does affect your score. BUT it also is a POSTIVE, under a manual review from other lenders. It is really important having a PAID installment loan. especially if paid on time in your credit history. This shows the lender you can take out a loan and pay them back with no trouble.
You also have to remember that your credit score is at it's most important when looking for a mortgage, auto loan, or other types of installment loans. Yes, I know that your credit score is looked at in other different situations but the most important times are when looking for an installment loan or some form, but lets be honest, getting a high enough score to qualify for the latest Amex card shouldn't really be on your higest priority list, And with that your past paid off loans are going to be viewed very positively.
So unless you are looking to get a line of credit, the temporary hit you will take is rather meaningless.
So I say if you can pay off your auto loan early and there are no early payment penelties than I say go ahead and pay it off and enjoy the savings you are about to receive, especially the future savings your are going to get by not paying interest on it.