What Is the Rule of 78s on a Car Loan?
Two people can pay off similar car loans a year early and end up with very different savings, and the reason often traces back to a decades-old interest formula called the rule of 78s.
The short answer
The rule of 78s is a method some precomputed-interest loans use to allocate interest across a loan’s term, front-loading a larger share of the total interest into the earliest payments rather than spreading it evenly. The name comes from the sum of the digits one through twelve, which totals 78 and forms the basis of the weighting formula for a twelve-month schedule. It matters mainly because it affects how much interest is considered “earned” if the loan is paid off before the end of its term.
How the weighting actually works
Instead of calculating interest fresh each month based on the current balance, as a simple-interest loan does, a rule-of-78s loan assigns a fraction of the total precomputed interest to each month based on its position in the schedule. Early months get a larger fraction and later months get progressively smaller ones, since the formula weights the sum of remaining months at each point. The practical effect is that a disproportionate share of the total interest is treated as earned in the first several payments, even though the outstanding balance hasn’t dropped much yet.
Why early payoff behaves differently under this method
Because more interest is front-loaded, paying off a rule-of-78s loan early typically returns a smaller rebate than a borrower might expect compared to a simple-interest loan with the same rate and term. Under simple interest, an early payoff mostly reflects interest that genuinely hasn’t accrued yet, since the calculation resets against the actual balance each day. Under the rule of 78s, a large chunk of the total interest has already been allocated to the months that passed, so there’s less left to rebate even if the loan is retired well ahead of schedule.
What to weigh if a loan uses this method
- Ask directly how interest is calculated. Loan documents or a truth-in-lending disclosure should indicate whether a loan uses simple interest, precomputed interest, or specifically the rule of 78s formula.
- Understand it mostly matters for early payoff. If a loan is carried to full term as scheduled, the total interest paid is generally the same regardless of allocation method; the difference shows up specifically when paying off early.
- Check for regulatory limits. Some states restrict or prohibit the rule of 78s on certain loan types or terms, so its use varies by jurisdiction and loan length.
- Compare against a straightforward alternative. A loan disclosed simply as simple-interest tends to be more transparent about what an early payoff actually saves, and pairing that with a plain amortization schedule removes the guesswork of decoding a weighting formula.
- Ask about a separate prepayment penalty. The rule of 78s is a distinct concept from an explicit auto loan prepayment penalty, since it reduces the early-payoff rebate through allocation rather than charging a standalone fee, but the practical effect on early payoff savings can feel similar.
Why this method still shows up at all
The rule of 78s predates the routine use of computers for daily interest calculations, when precomputing and allocating interest by a fixed formula was simpler to administer than recalculating a balance every day. It’s become less common in auto lending as simple-interest structures have become standard, but it can still appear in certain precomputed loan contracts, which is why checking the specific method matters more than assuming.
The bottom line
The rule of 78s allocates a loan’s total interest unevenly, weighting more of it toward the earliest payments, which reduces the benefit of paying off such a loan ahead of schedule compared to a simple-interest loan. Confirming which method a specific loan uses is the clearest way to know what an early payoff will actually save.