Cars & money, with the math written out

No hype, no affiliate links, no "it depends". Every page here answers one question a person actually types, shows the arithmetic, and says plainly where an example number came from.

All answers

• How much should I spend on a car? — the 20/4/10 rule, why insurance breaks it under 26, and a worked example.
• What a car build really costs — the $2,400 parts cart that lands at $4,078, itemised.
• Is a 72-month car loan bad? — 72 vs 48 months side by side, including how long you stay underwater.
• Snowball or avalanche: what does the choice actually cost? — $706 and two months against twenty months of feeling it work, simulated.
• Which card should I pay first to help my credit score? — the smallest-limit card, because a dollar moves utilization by 1 ÷ limit.
• How much does credit card interest cost per month? — $2.05 a day on $3,000 at 24.99%, and $62.48 of the first $100 you pay.
• How much faster does an extra $100 a month pay off a credit card? — from the minimum to a flat $200, 252 months becomes 47; each further $100 buys less.
• How much emergency fund do I need if I have a car payment? — size it to essentials with the car counted in full: 31.5% of the list on the example.
• Does the 50/30/20 rule work with a car payment? — needs are 69.3% of $3,600 with a $480 payment, 46.8% without the car. The rule diagnoses the car.
• How much should I save each month for car expenses? — $286.53 a month, two-thirds of it the insurance premium, on top of the payment and fuel.
• Why does my account go negative when I am under budget? — under budget and negative for 11 days: one six-month premium on the 4th, day by day.
• Is a car loan good debt or bad debt? — three years in, $6,395 of interest against $11,816 of depreciation. The car is the cost, not the loan.
• How much does a high-yield savings account actually earn? — $212 a year on $5,000, real and small; after three years of deposits, 92% of the balance is yours.
• How much do you save changing your own oil? — $47 a change, $845 a decade, and a first year that saves nothing once the tools are bought.
• How do I budget on an irregular income? — a fixed $3,200 salary from a buffer sized off the $2,100 floor month, simulated across a real-shaped year.