住宅・自動車decision

車購入適正予算シミュレーター (20/4/10ルール)

Never get trapped in underwater car debt. Test whether a new or used vehicle complies with the conservative 20/4/10 rule (20% down, 4-year loan, under 10% gross pay).

入力パラメータ

ブラウザ内処理のみ

計算結果と分析

計算準備完了 (¥0)

左側の入力フォームに数値を入力すると、リアルタイムで計算結果と推移グラフが生成されます。

20/4/10 Car Budget: $0 Limit

20/4/10 Safe Vehicle Price$0

Total Monthly Auto Cost$0Max Allowed: $0

Down Payment Saved0%

48-Mo Loan Payment$0

戦略的インサイト

主な洞察と次のアクション

The 20/4/10 rule prevents underwater auto debt: 20% down, 4-year max loan, under 10% gross income.
Car depreciation destroys wealth when financed over 60–84 month extended terms.
Enter your parameters above to determine your safe vehicle purchase ceiling.

セクション1: 計算の仕組み

この計算の仕組み（わかりやすい解説）

Car depreciation is brutal—most vehicles lose 20% of their value in the first year alone. The 20/4/10 rule protects buyers from being "underwater" on loans: (1) Put down at least 20% in cash. (2) Finance for no longer than 4 years (48 months). (3) Ensure total auto costs (loan payment + insurance + gas/maintenance) stay under 10% of your gross monthly income.

セクション2: 数式と計算前提

アルゴリズム数式と計算根拠

計算式:

\text{Down Payment} \ge 0.20 \times P, \quad T \le 48, \quad \text{Payment} + \text{Insurance} \le 0.10 \times Y_{\text{gross}}

モデルの前提条件:

Financing term is fixed at 48 months (4 years) to prevent negative equity.
Total cost of ownership includes insurance, fuel, and scheduled maintenance.

セクション3: 実践的なシミュレーション

Comparative Impact Modeling

戦略的な調整によって将来の推移がどう変わるか確認してください:

戦略 A

Baseline Minimum Strategy

Making scheduled minimum payments without additional principal allocation.

Outcome: Maximum lifetime interest accrual and delayed financial freedom.

戦略 B（最適化）

Accelerated Decision Strategy

Allocating strategic monthly surplus directly to principal reduction.

Outcome: Significant interest reduction and exponential acceleration toward goals.

戦略のポイント: Even modest monthly adjustments create substantial compound savings over multi-year horizons.

セクション4: 例外・境界条件

モデルの制限事項と境界条件

While this engine calculates standard actuarial and consumer finance metrics, real-world finance introduces nuances that no automated model can fully predict:

Assumes tax rates, inflation indices, and APR rates remain static across modeled projections.
Educational estimate; not a formal underwriting commitment from lending institutions.
Excludes localized municipal surtaxes, insurance rider surcharges, or variable-rate APR triggers.

よくあるご質問

よくある質問: 車購入適正予算シミュレーター (20/4/10ルール)

Why shouldn’t I take a 72-month or 84-month car loan to lower the payment?

Dealerships offer 72 to 84-month loans to artificially make expensive cars look affordable. On a 7-year loan, you will remain "underwater" (owing more than the car is worth) for 4 to 5 years, risking financial disaster if the car is totaled or breaks down.

Does the 20/4/10 rule apply to used cars?

Yes! In fact, the 20/4/10 rule works even better for 2–4 year-old certified pre-owned vehicles, where the original owner absorbed the steepest depreciation hit.