Your current interest rate is more than 1% higher than today's rates. Your credit score has improved since you got your original loan. You got your loan through the dealership, which typically marks up rates by 1-3%. You're struggling with monthly payments and need a longer term. Or rates have dropped since you financed.
Your loan balance is under $5,000 — the savings won't be significant enough. Your car is older than 10 years or has over 150,000 miles — most lenders won't refinance. You're near the end of your loan term — most of your remaining payments are principal, not interest. Or you have prepayment penalties on your current loan.
On a $25,000 loan, dropping from 8% to 5% saves about $40 per month and over $1,800 over the remaining term. The process typically takes a few days and most lenders handle the payoff of your old loan automatically.