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How long to wait before refinancing a high interest car loan, after improving credit history?
Between half a year and a year should be enough to improve your interest rates drastically on car loan refinance. Make sure that your new credit card has already been reported to the agencies, and that the credit/debt ratio is lower than 30% on your revolving (credit card) accounts. That also means that you shouldn't carry too much balance, even if the APR is 0%.