Why a car payment is not just the price divided up
A car loan is an ordinary fixed-rate loan, and the schedule underneath this page is the same one behind every other loan here. What makes a car deal its own question is everything that happens before the schedule starts. Between the price on the windscreen and the amount you actually borrow sit four things, and each of them moves the payment.
The trade-in comes off. Sales tax goes on. Registration and documentation fees go on. The deposit comes off. Change any one and the monthly figure changes, which is why a payment quoted without them is a number nobody can check.
The trade-in, and the part that catches people out
An old car is worth what the dealer allows for it. That allowance reduces what you borrow, in the ordinary case, and there is nothing surprising about it.
The surprising case is the common one. If the loan on the old car has not been paid down as fast as the car lost value, the payoff is larger than the allowance, and the difference does not disappear. It moves onto the new loan. You then borrow the price of the new car plus the shortfall on the old one, and you pay interest for years on a car you no longer own.
This page asks for the trade-in allowance and the payoff separately for exactly that reason. Entering only the allowance produces a payment that is too low, and it is too low in the direction that matters.
Sales tax: which basis, and why this page asks
Two states can charge the same rate on the same car and produce different bills. Most states that allow a trade-in credit tax the price less the allowance; others tax the whole price and treat the trade-in as separate. On a car with a substantial trade-in the gap between the two runs to real money.
The rate itself has to come from you. Sales tax on a vehicle is set by the state and very often by the county and the city on top, and no government body publishes those combined rates in one machine-readable list. Rather than invent a figure and dress it up as official, this page leaves the box to you and says plainly where the number is not coming from.
Financed or paid at signing
Tax and fees are due either way. The only question is whether they go onto the loan or come out of your pocket at the desk. Financing them raises the amount borrowed, so it raises both the payment and the interest; paying them at signing raises what you have to find on the day.
The total cost of the car is the same comparison made properly: everything paid at signing plus every instalment. It is the figure a monthly payment is worst at showing, and the reason a longer term can look cheaper every month while costing more in the end.
What this page does not model
It is a fixed-rate loan with level monthly payments and nothing else. There is no balloon payment, no interest-only period, no variable rate, no manufacturer subsidy and no cash-back offer weighed against a low rate. Leasing is a different arrangement with different arithmetic and is not what this page does.
Nor does it price insurance, extended warranties, service plans or gap cover. Those are commonly sold at the same desk and sometimes added to the same loan, but they are separate purchases and folding them into a car payment is how their cost stops being visible.