Vehicle EconomicsLarger Down Payment vs Keep Cash
Compare a guaranteed interest saving against a projected return on keeping the cash invested instead.
Why this comes up
You have cash beyond your planned down payment and a real choice: put it toward the car loan for a guaranteed interest saving, or keep it invested and hope the return beats what the loan is costing you. Both sides are real numbers, this engine puts the guaranteed saving next to the assumed return so the tradeoff is explicit instead of a gut call.
How we calculated this
We amortize the loan with and without the additional cash applied as down payment to get the guaranteed interest avoided. Separately, we project the same cash compounding monthly at your assumed annual return over the same term to get the retained-cash gain. We also solve the exact return rate where the two are equal.
Worked example, using this page's own defaults ($30,000 price, $5,000 additional cash, 7% APR, 60 months, 5% assumed return): the guaranteed interest avoided from a larger down payment tends to beat a modest assumed return at this APR, run the calculator above to check your own numbers.
What this means
- The interest saving from a larger down payment is guaranteed by the loan's own rate; the retained-cash gain depends entirely on an assumed return actually happening, they are not equally certain.
- A large down payment can also reduce negative-equity risk, a value the interest-only comparison above does not capture on its own.
- Never treat emergency savings as "additional cash" for this comparison, check your post-decision liquidity below before committing.
Limitations
The retained-cash return is whatever you assume, not a promised or historical rate; this engine does not model investment risk, taxes on gains, or account for the possibility the assumed return is not realized. See Methodology.
Methodology
This engine combines Vehicle Economics' fixed-rate amortization model (the standard loan-payment formula run out as a full month-by-month schedule) and break-even search model (a closed-form solve where one exists, otherwise a bounded search over a stated plausible range). See Methodology for the full detail.