If it's possible in your case to get such a loan, then sure, providing the loan fees aren't in excess of the interest rate difference. Auto loans don't have the fees mortgages do, but check the specific loan you're looking at - it may have some fees, and they'd need to be lower than the interest rate savings.
Car loans can be tricky to refinance, because of the value of a used car being less than that of a new car. How much better your credit is likely determines how hard this would be to get. Also, how much down payment you put down. Cars devalue 20% or so instantly (a used car with 5 miles on it tends to be worth around 80% of a new car's cost), so if you put less than 20% down, you may be underwater - meaning the principal left on the loan exceeds the value of the car (and so you wouldn't be getting a fully secured loan at that point).
However, if your loan amount isn't too high relative to the value of the car, it should be possible. Check out various lenders in advance; also check out non-lender sites for advice. Edmunds.com has some of this laid out, for example (though they're an industry-based site so they're not truly unbiased).
I'd also recommend using this to help you pay off the loan faster. If you do refinance to a lower rate, consider taking the savings and sending it to the lender - i.e., keeping your payment the same, just lowering the interest charge. That way you pay it off faster.