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The dealer asks "what payment do you want?" Ask the better question first.
Dealers sell payments because payments hide prices. This calculator works backward: the 20/4/10 rule (20% down, 4-year max loan, transport under 10% of gross income) with insurance and fuel already inside the cap — so the price it gives you is the whole-car answer, not a loan amount that forgets the insurance bill doubles on a new car.
Car shoppers use it before the dealership, because the dealer's first question ("what payment works?") is designed to hide the price. The 10%-cap-with-insurance-inside answer is the one no car site leads with.
20% down, a loan no longer than 4 years, and total transport costs — payment, insurance, fuel — under 10% of gross income. It is stricter than what lenders approve on purpose: cars depreciate roughly 20% in year one, so a long loan on a new car means years of owing more than the car is worth.
Because the dealer's math stops at the payment fitting your DTI — 72- and 84-month loans exist to make any price "fit." The 10% cap here includes insurance and fuel, which is where new-car budgets actually break. If the rule's price feels low, that is the information: the car market's norms are the outlier, not the rule.
At 0–3% promotional rates, stretching the term costs little in interest — but it still extends the underwater years, and one accident or job change while upside-down turns a car problem into a debt problem. If you must stretch, gap insurance and a bigger down payment are the seatbelts.
The rule works even better used: someone else paid the brutal first-year depreciation, reliability data exists, and insurance runs cheaper. A 3-year-old car at 65% of the new price with 80% of the life left is usually the value sweet spot.