Why monthly present in on line monthly payment calculator calculated as annual percent divided by 12? I think it's wrong, it should be exponential. For instance annual percent 4%. To calculate monthly percent we need to 1.04^(1/12). So monthly percent should be 0.327%, but if we use approach on on line calculators it is going to be 0.333.
Basically the way that APR is calculated varies depending on the industry and who quoted it. On a car they calculate the APR by taking the monthly percent and multiplying by 12, the calculator is actually right.
You probably want to calculate APY (annual percentage yield). Check this article out: http://www.finweb.com/loans/how-do-they-calculate-your-car-loan-apr.html#axzz3lTowC0op