Car TCO Skill

Cars are sold on monthly payments and owned on total cost — and the two rank options differently. Depreciation (the biggest cost of a new car) never appears on a statement; maintenance (the biggest fear about an old car) is usually smaller than a year of new-car depreciation. This skill compares buy-new, buy-used, lease-forever, and keep-current on the same total-cost basis over the same horizon.

What This Skill Produces

• Ranked scenario totals — every requested scenario over the same horizon, cheapest first
• True per-month cost — total ÷ months, the number to compare against the payment the dealer quotes
• The assumption ledger — depreciation curve, maintenance ramp, insurance deltas, all labeled
• The not-modeled list — financing interest, jurisdiction taxes, reliability luck

Required Inputs

Ask for these if not provided:

• Which scenarios to compare — any of: new price, used-equivalent price, lease terms, current car's value + annual maintenance
• Horizon — years they realistically keep cars (default 8, labeled); short horizons flatter leasing, long ones flatter buying
• Miles per year and rough fuel/energy cost (defaults 12,000 mi / $0.14 per mile, labeled)

Programmatic Helper

python3 scripts/car_tco.py --new-price 38000 --used-price 24000 --lease-month 420 --keep-value 9000
python3 scripts/car_tco.py --new-price 38000 --keep-value 9000 --keep-maint 1800 --horizon 6 --json

Deterministic. Depreciation: 20% year one then 10%/yr for new, gentler for used/current. Maintenance ramps 8%/yr (used cars start further up the ramp). Leases re-lease at each term end with a fresh drive-off. Resale value is credited back — TCO is what you spent minus what you can recover.

Framework: The Payment Illusion Rules

• Depreciation is the invisible line item — a new car's largest cost has no bill; it's the resale-value credit shrinking
• "My old car needs $2,000 of work" is usually cheap — compare the repair to a year of the replacement's depreciation before calling it "not worth fixing"
• Leasing buys flexibility, not savings — lease-forever means paying peak depreciation years forever; price that honestly and let people buy flexibility knowingly
• Margins under ~10% are noise — the model's assumptions can't resolve differences that small; say "roughly a tie," not a winner
• The horizon drives the ranking — always name it; the same inputs rank differently at 3 years vs 10

Output Format

Car TCO: [scenarios] over [N] years

The Ranking

[Script output: totals cheapest-first, per-month true cost]

Reading the Ranking

[Two sentences: how far apart the options really are, and which assumption could flip the order.]

What This Model Ignores

Financing interest (run the loan separately) · taxes and fees by jurisdiction · reliability variance (a lemon breaks any model) · the non-financials (safety features, the want).

Educational model, not financial advice — verify with a licensed professional before acting on it.

Quality Checks

• All scenarios use the same horizon and mileage
• Resale value is credited back in every ownership scenario
• Per-month true cost appears next to the totals
• Sub-10% margins are called ties, not winners
• The disclaimer line appears in the artifact

Anti-Patterns

• Do not rank on monthly payment — that's the illusion this skill exists to correct
• Do not omit depreciation or resale from ownership scenarios
• Do not treat maintenance fears as data — use the ramp, and compare repairs to replacement depreciation
• Do not extrapolate one scenario's horizon onto another (a 3-year lease vs 8-year ownership is not a comparison)
• Do not moralize the want — price the options honestly and let the user choose with open eyes