The best time to refinance a car is when you can secure better loan terms than your current one. This often includes when market interest rates have dropped, or your credit score has improved since you first took out the loan. Other good times to refinance include when you have positive equity in the car (the car is worth more than you owe) or if you want to reduce monthly payments to better fit your budget. Also, refinancing can be a good option if you are unhappy with your current lender's service. However, you should check if your current loan has any prepayment penalties to ensure refinancing saves you money overall.

Generally, you can refinance as soon as you qualify for better terms, but it's wise to do so early enough in your loan term to make the switch worth the costs involved.