Two curves, one race
Every used-car loan is a race between two curves. The collateral curve falls: the method models wholesale value as straight-line depreciation from today's wholesale value down to a $1,000 scrap floor, spread over the vehicle's remaining useful life (floored at one year). Monthly depreciation is simply (wholesale value − $1,000) ÷ (12 × years of useful life remaining). The balance curve also falls, but on a different schedule: a standard fully amortizing payment, where the outstanding balance after m months is the compounded principal minus the compounded value of payments made.
The underwater point is the first month where projected balance exceeds projected wholesale value. At subprime rates in the low twenties, early payments are mostly interest, so the balance curve starts flat while the collateral curve falls at full speed. That asymmetry is the entire mechanism. And when the amount financed already sits above wholesale value at signing, because of rolled negative equity or add-ons in the note, the loan is underwater from month one and the race is over before it starts.
How long a car actually lasts
The method groups vehicles into six reliability tiers, each with a useful-life ceiling defined as a mileage limit or an age limit, whichever comes first, a reliability multiplier applied to repair hazard, and an average major-repair cost. The tier ordering and its ceilings are not invented and are not drawn from any proprietary or vehicle-history dataset: they are calibrated to public, published data. The iSeeCars Longest-Lasting Cars Study (2025), built on roughly 400 million vehicles, ranks Toyota, Lexus, Honda, and Acura as the only brands whose models clear the 4.8% industry-average odds of reaching 250,000 miles, which is why they anchor Tier 1. The age ceilings track the U.S. Bureau of Transportation Statistics average-age-of-vehicles-in-operation series, which puts the typical light vehicle on the road well past a decade old, and the repair-cost column follows the brand ordering in RepairPal and Consumer Reports cost-of-ownership data, where European luxury marques run roughly two-to-four times mainstream annual repair cost. Sources are listed in full under Data & sources.
| Tier | Representative makes | Useful life | Hazard multiplier | Avg major repair |
|---|---|---|---|---|
| 1 | Toyota, Honda, Mazda | 250k mi / 18 yr | 0.70× | $1,800 |
| 2 | Hyundai, Kia, Subaru, Lexus, Nissan | 220k mi / 16 yr | 0.85× | $2,000 |
| 3 | Ford, Chevrolet, Ram pickups | 220k mi / 16 yr | 0.95× | $2,200 |
| 4 | Ford, Chevrolet sedans and SUVs | 190k mi / 14 yr | 1.00× | $2,100 |
| 5 | BMW, Mercedes, Audi | 150k mi / 12 yr | 1.20× | $4,000 |
| 6 | Land Rover, Jaguar, older Volvo | 130k mi / 10 yr | 1.35× | $4,500 |
Major-repair risk is modeled as a per-year hazard rate that steps up with the vehicle's age at the start of each loan year, then gets multiplied by the tier's hazard multiplier and capped at 85% per year: 5% under age 5, 15% at ages 5-7, 30% at 8-10, 45% at 11-13, and 60% at 14 and older. Cumulative probability over the term is one minus the product of the yearly survival rates, and expected repair burden is that cumulative probability times the tier's average repair cost.
The recommended-max-term rule
The rule is one line. Compute the vehicle's remaining runway two ways: by age (useful-life years minus current age) and by mileage (remaining miles divided by expected annual mileage, defaulting to roughly 13,500 miles a year per Federal Highway Administration figures unless the applicant's own usage says otherwise). Take the smaller of the two, convert to months, and subtract a 12-month buffer so the loan retires while the borrower still has a functioning car and trade-out options. The result is clamped between 12 and 84 months. The same runway also splits the term into condition zones: the first 55% of remaining useful life is treated as healthy, 55-85% as aging, and everything beyond 85% as end-of-life.
The verdict bands follow directly. Matched: the proposed term is at or under the recommended maximum, and the vehicle stays inside both its age and mileage ceilings through maturity. Risky: the term overshoots the recommended maximum by 12 months or less but the vehicle still finishes inside its useful life. Mismatched: anything longer, or any structure where projected age or mileage at maturity exceeds the ceiling, meaning the loan is scheduled to outlive the car. Where a deal lands risky or mismatched, the arithmetic offers three repairs: shorten the term to the recommended maximum, add roughly 15% of the amount financed as additional money down (with a $500 minimum) so a shorter term carries the same payment, or put the borrower in a vehicle with more runway.
Worked example: a ten-year-old sedan on 72 months
All numbers in this section are synthetic and illustrative, generated from the formulas above. Take a tier-4 domestic sedan: 10 years old, 120,000 miles, $6,000 wholesale value, driven 12,000 miles a year. Proposed structure: $7,500 financed over 72 months at 24% APR, payment about $197. The runway math: 4 years left by age, 5.8 by mileage, so 4 years governs; 48 months minus the 12-month buffer gives a recommended maximum term of 36 months. The proposed 72-month term doubles it, and the car crosses its 14-year ceiling at month 48 with two years of loan left.
| Month | Projected balance | Projected wholesale value | Equity gap |
|---|---|---|---|
| M0 | $7,500 | $6,000 | −$1,500 |
| M12 | $6,864 | $4,750 | −$2,114 |
| M24 | $6,056 | $3,500 | −$2,556 |
| M36 | $5,033 | $2,250 | −$2,783 |
| M48 | $3,735 | $1,000 (scrap floor) | −$2,735 |
| M60 | $2,088 | $1,000 | −$1,088 |
| M72 | $0 | $1,000 | +$1,000 |
At the recommended 36-month term the same $7,500 amortizes at about $294 a month, roughly $97 more than the 72-month payment. That is the honest trade on the table: the long term does not make the deal affordable, it moves the cost from the payment line into a 98% chance of a repair-versus-default decision on a car the borrower cannot exit.
The negative-equity window
What matters is not just whether the loan goes underwater but how long it stays there, because that window is when a breakdown converts into a default. The method reads the trade-out gap, balance minus value, in four bands. More than $2,000 underwater: the borrower is stuck; no trade-in covers the shortfall, and no rational replacement loan absorbs it. Up to $2,000 underwater: barely movable, and only by rolling the gap into a worse next loan. Thin equity under $2,000: one major repair wipes it out. Real equity beyond that: the borrower can trade, refinance, or absorb the repair. In the worked example the borrower sits in the stuck band from roughly month 6 through month 52, which is most of the contract.
Limits
This method cannot see the individual unit. A meticulously maintained ten-year-old car and an abused one carry the same tier, age, and mileage inputs; the hazard schedule prices the class average, not the vehicle in front of you. Straight-line depreciation ignores used-market cycles, so in a year when wholesale values swing sharply, the value curve will be wrong in whichever direction the market moved. The tier ceilings compress wide model-level variance into six buckets, the $1,000 scrap floor is an assumption, and annual mileage is whatever the applicant reports. Most importantly, the hazard rates and useful-life thresholds are modeled calibrations to public reliability studies, not parameters fitted to loan-level outcome data. The method flags a structural mismatch between term and collateral; it does not predict any specific loan's fate, and it says nothing about income shocks, which end loans regardless of equity position.