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
Anti-Patterns
1---2name: car-tco3description: Compare the total cost of car ownership across buy-new, buy-used, lease, and keep-your-current-car — depreciation, insurance, maintenance ramp, and fuel over a real horizon, not just the monthly payment. Use when asked should I lease or buy a car, is it cheaper to keep my old car, what does this car really cost per month, or new vs used total cost. Produces the ranked scenario totals, per-month true cost, the assumption ledger, and the not-modeled list.4---5
6# Car TCO Skill
7
8Cars 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.
9
10## What This Skill Produces
11
12- **Ranked scenario totals** — every requested scenario over the same horizon, cheapest first
13- **True per-month cost** — total ÷ months, the number to compare against the payment the dealer quotes
14- **The assumption ledger** — depreciation curve, maintenance ramp, insurance deltas, all labeled
15- **The not-modeled list** — financing interest, jurisdiction taxes, reliability luck
16
17## Required Inputs
18
19Ask for these if not provided:
20- **Which scenarios to compare** — any of: new price, used-equivalent price, lease terms, current car's value + annual maintenance
21- **Horizon** — years they realistically keep cars (default 8, labeled); short horizons flatter leasing, long ones flatter buying
22- **Miles per year** and rough fuel/energy cost (defaults 12,000 mi / $0.14 per mile, labeled)
23
24## Programmatic Helper
25
26```bash
27python3 scripts/car_tco.py --new-price 38000 --used-price 24000 --lease-month 420 --keep-value 9000
28python3 scripts/car_tco.py --new-price 38000 --keep-value 9000 --keep-maint 1800 --horizon 6 --json
29```
30
31Deterministic. 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.
32
33## Framework: The Payment Illusion Rules
34
35- **Depreciation is the invisible line item** — a new car's largest cost has no bill; it's the resale-value credit shrinking
36- **"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"
37- **Leasing buys flexibility, not savings** — lease-forever means paying peak depreciation years forever; price that honestly and let people buy flexibility knowingly
38- **Margins under ~10% are noise** — the model's assumptions can't resolve differences that small; say "roughly a tie," not a winner
39- **The horizon drives the ranking** — always name it; the same inputs rank differently at 3 years vs 10
40
41## Output Format
42
43---
44
45# Car TCO: [scenarios] over [N] years
46
47## The Ranking
48[Script output: totals cheapest-first, per-month true cost]
49
50## Reading the Ranking
51[Two sentences: how far apart the options really are, and which assumption could flip the order.]
52
53## What This Model Ignores
54Financing interest (run the loan separately) · taxes and fees by jurisdiction · reliability variance (a lemon breaks any model) · the non-financials (safety features, the want).
55
56*Educational model, not financial advice — verify with a licensed professional before acting on it.*
57
58---
59
60## Quality Checks
61
62- [ ] All scenarios use the same horizon and mileage
63- [ ] Resale value is credited back in every ownership scenario
64- [ ] Per-month true cost appears next to the totals
65- [ ] Sub-10% margins are called ties, not winners
66- [ ] The disclaimer line appears in the artifact
67
68## Anti-Patterns
69
70- [ ] Do not rank on monthly payment — that's the illusion this skill exists to correct
71- [ ] Do not omit depreciation or resale from ownership scenarios
72- [ ] Do not treat maintenance fears as data — use the ramp, and compare repairs to replacement depreciation
73- [ ] Do not extrapolate one scenario's horizon onto another (a 3-year lease vs 8-year ownership is not a comparison)
74- [ ] Do not moralize the want — price the options honestly and let the user choose with open eyes
Mohit Aggarwal