Chunking only saves interest when the loan charges interest on what you actually owe. A chunk is a lump sum you move from a line of credit onto a loan's principal, and on most car loans it works exactly the way you'd hope. Those are simple-interest loans, which means interest is figured on the remaining balance each day, so every extra dollar of principal stops future interest the day it lands.
Some car loans work differently: on a precomputed loan, the lender figures the interest for the entire term at signing and adds it to what you owe. You'll see them most often at buy-here-pay-here lots and with some lenders who work with weaker credit. Extra payments on a precomputed loan usually don't lower the interest along the way. They're credited as payments made early, and the interest only changes when you pay the whole loan off, at which point you get a refund of the interest that hadn't been earned yet.
The contract decides how that refund is figured. Some use the actuarial method, which comes out close to simple interest. Others use the Rule of 78s, which counts more of the interest as earned in the early months, so the refund is smaller. Federal law bans the Rule of 78s on consumer loans longer than 61 months, and some states limit it further, but a shorter loan can still carry it.
Take an $18,000 loan at 18% for 60 months, with a payment of about $457. Pay it off after a year, and the payoff on a simple-interest loan is about $15,560. Under the Rule of 78s, it's about $15,883, or $323 more. That difference isn't huge. The bigger problem is partial chunks. Send $3,000 against a precomputed loan, and you may save nothing at all until the last dollar is paid.
So check before you chunk. The retail installment contract usually says "simple interest" or "precomputed" in its terms, and it explains how any refund is calculated. In the truth-in-lending box, look at the prepayment line. If it says you may be entitled to a refund of part of the finance charge, the interest was figured up front. If it's still unclear, call the lender, ask for a payoff quote dated a month out, and ask how an extra payment would be applied. On a simple-interest loan, make sure extra money goes to principal and isn't just counted as next month's payment made early. While you have the contract out, look for a prepayment penalty too, the same check covered in checking for a prepayment penalty.
If the car loan turns out to be precomputed, don't chunk it a piece at a time. Aim your chunks at the simple-interest debts first, the way chunking car loans and student debt lays out, and pay the precomputed loan off in one shot from the line of credit when the payoff quote makes sense.
Pull the car loan contract out tonight and read the prepayment line before your next chunk goes anywhere. A promised refund of part of the finance charge means the interest was figured at signing, and a partial chunk won't lower it.