How we compute true cost
1. Fair price
A gradient-boosted model (LightGBM) trained on 137552 live UK listings (AutoTrader, cinch, Cazoo, drive.co.uk — September 2026) prices every advert against the market: make, model, variant text, age, mileage, fuel, transmission, body type and more. Holdout accuracy: RMSLE 0.161, mean absolute error ~£1,770. Your asking price ÷ our fair price gives the deal grade: A ≤0.90 · B ≤0.97 · C ≤1.03 · D ≤1.10 · E above. Listings below 0.70 are quarantined (grade X) — usually damaged or category-writeoff cars.
2. Resale forecast
The same model rolled forward: +24/36/48 months of age and +10,000 miles/year, minus a 6% selling-cost haircut. Backtested by training on October 2022 data and predicting September 2026 values for 770 make-model cohorts: median error ~24% over four years including market drift (16.5% for 3–4-year-old cars). Treat multi-year forecasts as a central estimate with wide uncertainty.
3. Finance simulation
For each car we compute, per payment method, the money in minus money out — depreciation + interest + fees — as £/month:
| Method | Model | Rate used |
|---|---|---|
| Cash | price − resale, plus 4% foregone savings | 4% AER |
| Personal loan | 10% deposit, amortised balance | 7.4% APR |
| Hire purchase | amortised + £100 option fee | 8.9% APR |
| PCP | balloon at 95% of forecast resale; handback vs buy-and-sell both computed, cheaper exit shown | 7.9% APR |
| Lease | market monthly rates from comparison feeds | per deal |
Rates are representative market averages, not offers. Your credit will change them. Excess mileage on PCP/lease charged at £0.10/mile over 10,000/yr.
4. What's not included (yet)
Insurance, road tax (VED), fuel/electricity, servicing and MOT are not in v1 figures — they arrive in the running-cost layer. They change the level, much less the ranking between similar cars.
Estimates for information only — not financial advice.