It all depends on the new rate you would get, the loan fees, and (most importantly) your balance on the loan. Going from 8% to 6.7% on a balance of $15,000 is a much bigger deal than doing so on $7,000.
If you're interested in saving money, you're probably better off paying an extra $100 or even $50 a month on the car. That will likely save you more interest--especially if you're early-on in the course of the loan--than shaving a percentage point or so off the interest.
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in a credit-scoring postnuclear Stone Age...