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Optimal down payment amount
The optimal down payment is 0% IF your interest rate is also 0%. As the interest rate increases, so does the likelihood of the better option being to pay for the car outright. Note that this is probably a binary choice. In other words, depending on the rate you will pay, you should either put 0% down, or 100% down. The interesting question is what formula should you use to determine which way to go? Obviously if you can invest at a higher return than the rate you pay on the car, you would still want to put 0% down. The same goes for inflation, and you can add these two numbers together. For example, if you estimate 2% inflation plus 1% guaranteed investment, then as long as the rate on your car is less than 3%, you would want to minimize the amount you put down. The key here is you must actually invest it. Other possible reasons to minimize the down payment would be if you have other loans with higher rates- then obviously use that money to pay down those loans before the car loan. All that being said, some dealers will give you cash back if you pay for the car outright. If you have this option, do the math and see where it lands. Most likely taking the cash back is going to be more attractive so you don't even have to hedge inflation at all. Tip: Make sure to negotiate the price of the car before you tell them how you are going to pay for it. (And during this process you can hint that you'll pay cash for it.)