When using an auto loan finance calculator, it is important to consider the variables that all simple interest calculators use. First of all, unless you are using a financial calculator with a tax function or a spreadsheet that includes a cell for tax, your monthly payments will not include sales tax. In most states, tax is based on the "top line" selling price of the vehicle before rebates. So if you would like to calculate tax, just multiply the selling price by your state tax rate and add it to the selling price.
Online finance calculators and loan charts also allow you to choose an approximate interest rate to use for calculations. While this will allow you to get a gauge for car payments at various interest rates, the range of rates you use is not indicative of what you may qualify for. Websites like Bankrate.com and CapitalOne can provide information on approximate interest rates for borrowers in a variety of credit situations. You can then use these approximations to properly estimate your payment using an auto loan finance calculator.
Additionally, most auto loan calculators omit charges like dealer documentation fees, license fees, title fees, luxury taxes and emissions certification fees. If you choose to finance these charges rather than pay cash, your monthly payments will be higher than the payment calculator states. You can include these charges in a finance quote by raising your selling price by the appropriate amount. These fees are not normally taxed. You can check your state's motor vehicle department or contact your dealership to get an idea of what these fees may be.