Basically, an auto loan is a long-established account that you have (hopefully) kept in good standing—which is a positive sign for future lenders and the FICO credit score formula. But once you close out the loan account, the average age of your credit history and your mix of use may drop.
In other words, according to the FICO credit score formula, a loan that’s almost paid off is better for your credit than one that’s completely paid off. That being said, the hit to your credit score is likely to be mild, and it shouldn’t impact your ability to qualify for loans in the future.