How the numbers are built
Sales tax is applied to the price minus your trade-in (the rule in most states), then the amount financed is price + tax − down payment − trade-in. The payment uses standard amortization. Watch the total-cost line: a $35,000 car at 7% for 72 months quietly becomes roughly $45,000 with tax and interest.
The 20/4/10 rule of thumb
A classic affordability guardrail: put at least 20% down, finance for no more than 4 years, and keep all car costs (payment, insurance, fuel) under 10% of gross income. Few buyers hit all three today — the average new-car loan is over 68 months — but every step toward the rule cuts interest and slashes the risk of owing more than the car is worth.
Why long loans and low payments are a trap
Dealers sell payments, not prices, because stretching the term hides cost. Going from 60 to 84 months on $30,000 at 7% drops the payment about $141 but adds roughly $2,400 in interest — and cars depreciate ~20% in year one, so long loans spend years underwater. Negotiate the out-the-door price first, arrange financing (or a credit-union preapproval) separately, and only then discuss monthly payments.
Common mistakes at the dealership
The costliest errors happen after you've picked the car. Negotiating from the monthly payment lets the dealer stretch the term and bury the real price — always negotiate the out-the-door total instead. Rolling negative equity from your trade-in into the new loan starts you underwater on day one. Financing add-ons like extended warranties, paint protection, and gap insurance at the finance desk inflates the amount borrowed and the interest on it; you can usually buy the ones you actually want cheaper elsewhere. And accepting dealer financing without a competing quote costs many buyers a point or two of rate — get a credit-union preapproval first and make the dealer beat it.