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The best way to pay extra is to pay extra. If you pay 608 a month you will reduce your loan term by 18 months and save 4000 in interest. (paying 700 will cut 28 months off and save 6k in interest)
A CU might help depending upon your scores, but you will definitely need to pay extra each month. At 17.9% it is easy to get underwater on your loan, and if you owe too much compared to the value of the vehicle no one will touch it. Paying extra, especially upfront will help you keep ahead.
808 would cut you down from 72 months to 36 months and save you about 7500 in interest.