Simple Interest vs Pre-Computed Auto Loans: The Buy-Here-Pay-Here Trap Hidden in Plain Sight
Simple-interest loans calculate interest daily on the remaining balance. Pre-computed (add-on) loans calculate ALL interest upfront. The math difference: $800-$3,200 in extra interest over a typical loan.
Quick answers
- Does paying extra principal hurt me on a pre-computed loan?
- No — but it doesn't help much either. Extra principal on a pre-computed loan reduces the loan term but doesn't reduce total interest. You just finish paying sooner.
- Can a dealer convert my pre-computed loan to simple-interest?
- No — the loan structure is determined at origination. To switch, you must refinance into a new loan with a different lender.
- What's the Rule of 78s?
- A method of calculating interest payoff on pre-computed loans that heavily front-loads interest in early months. If you pay off a Rule of 78s loan after 24 of 60 months, you've already paid roughly 60% of the total interest — not 40% as you'd expect.
The two interest models
Auto loans use one of two interest calculation methods. The federal Truth-In-Lending Act requires disclosure of both, but the disclosure language is technical and most borrowers don't catch the difference.
Simple Interest (the good kind): Interest accrues DAILY on the remaining loan balance. As you pay down principal, your daily interest decreases. Paying extra principal directly reduces future interest.
Pre-Computed / Add-On Interest (the bad kind): ALL interest for the loan term is calculated upfront and added to the principal. Each monthly payment splits between principal and interest in a fixed ratio. Paying extra DOES NOT reduce total interest; it just shortens the loan term.
The math difference
$25,000 loan, 7.5% APR, 60-month term:
Simple interest:
- Total interest: $5,026 (assumes on-time payments)
- Monthly payment: $501
- Early payoff penalty: NONE — saves remaining interest
Pre-computed interest:
- Total interest: $5,026 (same baseline)
- Monthly payment: $501
- BUT if you pay $5,000 extra at month 24: interest savings = ~$0 (the interest was already pre-computed)
If you pay extra principal on a pre-computed loan, you don't save interest — you just shorten the term. The lender keeps the interest they pre-computed.
Where each loan type lives
Simple interest (most loans):
- All bank loans (Bank of America, Wells Fargo, Chase)
- All credit union loans (Navy Federal, PenFed, local CUs)
- Online aggregators (AutoPay, Caribou, LightStream)
- Most captive manufacturer financing (Toyota, Honda, Ford)
- Even most BHPH dealers in 2025+ (regulations forced the shift)
Pre-computed interest (loans to avoid):
- A small subset of subprime / BHPH (buy-here-pay-here) dealers, especially in TX, OK, AR, MO
- Some title-loan lenders (Westlake at the lowest credit tiers)
- Older / legacy loans originated before 2020 in certain states
How to tell which you have
Check page 3 of your loan agreement. Look for the disclosure language:
- "Interest accrues daily on the unpaid principal balance" = SIMPLE INTEREST ✓
- "Total finance charges of $X have been pre-computed" = PRE-COMPUTED ✗
- "Rule of 78s" payoff method = PRE-COMPUTED ✗ (worst variant)
The Rule of 78s is especially predatory: it front-loads interest so that early payoffs return very little interest savings. If you see "Rule of 78s" anywhere in your loan documents, you have a pre-computed loan with the worst possible payoff curve.
The escape route
If you have a pre-computed loan, the only way to save interest is to refinance into a simple-interest loan. The savings on a typical $25k pre-computed loan with 36 months remaining: $1,200-$2,800 in avoided interest. Refinancing fees: $75-$200. Net: $1,000-$2,600 saved.
Rates as of Jun 7, 2026
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Comparing 5 lenders· Rates verified Jun 7
Data last reviewed . Source: CarSavr editorial methodology.
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| Lender | Loan amount | Term | ||||
|---|---|---|---|---|---|---|
1 LightStream | 6.94–14.94% Total int. ~$4,659 · $25k · 60mo | 660+ | $5K–$100K | 24–84 mo | Reviewed today | |
2 AutoPay Best marketplace | 5.69–17.99% Total int. ~$3,783 · $25k · 60mo | 580+ | $5K–$100K | 24–84 mo | Reviewed today | |
3 PenFed Credit Union Best credit union | 5.24–17.99% Total int. ~$3,472 · $25k · 60mo | 610+ | $500–$150K | 36–84 mo | Reviewed today |
- APR
- 6.94–14.94%
- Min. credit
- 660+
- Loan amount
- $5K–$100K
- Term
- 24–84 mo
- APR
- 5.69–17.99%
- Min. credit
- 580+
- Loan amount
- $5K–$100K
- Term
- 24–84 mo
- APR
- 5.24–17.99%
- Min. credit
- 610+
- Loan amount
- $500–$150K
- Term
- 36–84 mo
APR ranges are sourced from each lender's public site and are updated regularly. Your actual rate depends on credit history, loan amount, vehicle, and state. CarSavr may earn a commission when you apply through our links — it never affects how we rank lenders.
Provider logos and trademarks belong to their respective owners and are used for identification purposes only. Providers shown for comparison and educational purposes — display does not imply partnership unless an active affiliate relationship is stated separately.
How rows are ranked: Editor's pick first, then by overall rating. Promoted placements are flagged with a Sponsored badge. Read the full methodology →
Lenders that will refinance you out of a pre-computed loan: AutoPay, Caribou, LendingClub, your local credit union, and most online aggregators.
How to avoid pre-computed loans at purchase
- Read page 3 of the loan agreement BEFORE signing
- Ask the F&I officer directly: "Is this a simple-interest loan?"
- Look for the magic phrase "interest accrues daily on the unpaid principal"
- Walk away from any dealer that won't show you the calculation method clearly
- Get pre-approved from a bank or credit union before walking into the dealer — those are virtually always simple-interest
FAQs
Does paying extra principal hurt me on a pre-computed loan?
No — but it doesn't help much either. Extra principal on a pre-computed loan reduces the loan term but doesn't reduce total interest. You just finish paying sooner.
Can a dealer convert my pre-computed loan to simple-interest?
No — the loan structure is determined at origination. To switch, you must refinance into a new loan with a different lender.
What's the Rule of 78s?
A method of calculating interest payoff on pre-computed loans that heavily front-loads interest in early months. If you pay off a Rule of 78s loan after 24 of 60 months, you've already paid roughly 60% of the total interest — not 40% as you'd expect.
Is precomputed interest illegal?
It's legal at the federal level but restricted or banned in several states (Massachusetts, Connecticut, Pennsylvania, Rhode Island). The Consumer Financial Protection Bureau actively reviews it but hasn't banned it nationally.
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- Auto Loan Hardship Programs: What 12 Major Lenders Actually Offer (and the 3-Step Approval Process)
Terms in this article
4 financial terms defined
Auto Loan
A secured installment loan used to purchase a vehicle, with the car serving as collateral.
Auto LoansAPR (Annual Percentage Rate)
The yearly cost of a loan including interest and fees, expressed as a percentage.
Auto LoansRefinance
Replacing your current auto loan with a new loan at better terms.
Auto LoansF&I (Finance & Insurance Office)
The dealer office that handles loan paperwork and sells add-on products.
Ownership & PricingSee if you're overpaying
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