Work out your monthly car payment, accounting for trade-in value, down payment, and sales tax.
Sales tax is applied to the price minus your trade-in value, then added to the loan. The remaining balance amortizes at your interest rate over the loan term, same as any fixed-rate loan.
Most US states only charge sales tax on the price minus your trade-in value, which is why trading in a vehicle can meaningfully lower your total loan amount even before considering the trade-in credit itself.
In most US states, yes — sales tax is calculated on the price after subtracting your trade-in value, not the full vehicle price.
Add them to the vehicle price field for the most accurate financed amount, since they're typically rolled into the loan.