Compare the loan rate to what your cash safely earns — not to what stocks might return
If you have enough cash to buy something outright, financing it is a choice to keep your money and rent someone else's. That is not automatically wrong. Whether it is right comes down to one comparison, and most people make it against the wrong benchmark.
The common framing is: my loan is at 6 percent, stocks historically return about 10 percent, therefore financing wins and I invest the difference. This reasoning is seductive and it is unsound, because it compares a certain obligation to an uncertain return.
The loan payment is fixed, contractual, and due every month whatever happens. The market return is an average across decades containing long stretches of loss. Trading a guaranteed cost for an expected return is taking on risk, and it should be priced as risk rather than treated as free money.
Compare the loan APR against what your cash earns with comparable certainty. That is the risk-free rate — Treasury bills, a high-yield savings account, a money market fund — not the historical equity return.
If your cash can safely earn more than the loan costs, financing is genuinely profitable and the gap is real money. If the loan costs more than your cash safely earns, financing costs you the difference, and the case for it has to be made on other grounds.
Those other grounds exist and can be perfectly good. Cash you keep is available for emergencies, which has real value that no interest rate captures. A promotional low rate from a manufacturer can beat any savings account. And there are situations where preserving liquidity matters more than the spread, particularly if your income is unstable.
One thing to watch: a manufacturer's subsidised rate is frequently offered as an alternative to a cash rebate, not in addition to it. If taking the low APR costs you a discount on the price, the true cost of that financing is the rate plus the rebate you gave up — which can make a "0 percent" loan considerably more expensive than it looks.
Benchmark against the risk-free rate, not expected stock returns
Personal auto and consumer loan interest is not tax-deductible, so compare after-tax cash yield to the full APR
A subsidised APR offered instead of a rebate is not actually free
Cash retained has option value: it can absorb an emergency, and a car cannot
Paying cash removes a fixed obligation, which lowers risk regardless of the arithmetic
For a long stretch after 2008, short-term safe rates were close to zero. In that environment financing at almost any rate cost real money, because cash sitting in savings earned essentially nothing — and the "invest the difference" argument was doing all the work.
That is not a permanent condition. Short-term Treasury yields have moved through wide ranges historically, and there have been extended periods when safe cash yielded more than a good auto loan rate. In those conditions financing is straightforwardly profitable and paying cash is the expensive choice.
Which means this question has no fixed answer. It depends on the spread between two numbers you can look up in about a minute: your quoted APR, and the current yield on short-term Treasuries or a high-yield savings account. Anyone who tells you financing is always smart, or that debt is always to be avoided, is answering a question about temperament rather than arithmetic.
Check the 3-month Treasury yield and the FDIC national deposit rates before you decide. Both are published, both are free, and the gap between them and your quoted APR is the entire financial content of this decision.
Preserves liquidity, which is what actually absorbs an emergency
Profitable outright when safe cash yields exceed the loan rate
A genuinely subsidised manufacturer rate can be very cheap money
Spreads a large outlay without draining your reserves
On-time payments build credit history
A fixed obligation that survives job loss, illness, and every other change in circumstance
Interest on personal vehicle loans is not deductible, so the full rate is the real cost
Combined with front-loaded depreciation, it can leave you underwater and unable to sell
A low advertised rate may be purchased with a forfeited rebate
Lender requirements can force coverage you would not otherwise buy
Lean toward cash if your income is unstable: The value of not owing anyone a fixed payment rises sharply when your income might fall. This is a risk decision, not a yield decision.
Lean toward financing if paying cash empties your reserves: Buying outright and leaving nothing behind converts a manageable monthly cost into a genuine emergency the first time something breaks.
Run the numbers if the spread is close: Within a point or so either way, the financial difference is small enough that liquidity and peace of mind should decide it.
Always check what the low rate costs: If a subsidised APR is offered instead of a rebate, compute both deals to the total dollars paid before choosing.
Get a rate before you shop: A pre-approval from your own bank or credit union gives you a real benchmark and removes the seller's ability to present financing as a package deal with the price.
Look up what your cash earns: Check the current 3-month Treasury yield and the FDIC national savings rates. That is your comparison figure, and it takes a minute.
Compute both deals to total dollars: Cash price versus financed total including all interest, and if a rebate is in play, run the low-APR-without-rebate option against the high-APR-with-rebate option explicitly.
Ask what happens if income stops: Whichever way the arithmetic points, check the answer against a bad scenario. If a lost job makes the payment untenable within two months, the spread is not the deciding factor.