This calculator allows you to include some of the adjustments that might impact the financing of your car:
If you have a car to trade in then you should make an estimate of its trade in value.
Assuming you still have a loan on the car that you are trading in, you should insert the amount that it would cost to pay off that loan.
If you have any savings that you want to put towards the cost of a new car then add this amount as a cash deposit paid.
Use the number of months to adjust the loan period. A longer loan period - 72 months is about as long as can go - will make the monthly payments lower.
Finally, add the interest rate that you thing you can get on a car loan. Remember that longer loan periods typically incur higher interest rates.