Buying a Car: How to Get the Numbers Right

Start with affordability, not the car

The single most expensive mistake in car buying is choosing the car first and working out the money second. Decide what you can afford before you look at a single listing, and you immune yourself to the upsell. A common guardrail is the 20/4/10 rule: put at least 20% down, finance for no more than four years, and keep total car costs — payment, fuel, insurance, maintenance — under 10% of gross income.

That rule deliberately produces a smaller number than a lender will offer. A bank approves you for the largest payment your income can technically service; the 20/4/10 figure is the one that leaves room for rent, savings, and a life. Set the band, then shop inside it.

New vs used: the depreciation trade-off

A new car loses value the moment it leaves the lot, and keeps shedding it fastest in the first two to three years — often 40% or more of its price by year three. That lost value is real money, and it’s the single biggest cost of owning a new car. Someone has to pay it.

Buying a two- or three-year-old car lets the first owner absorb that initial hit while you get most of the useful life. The trade-offs are a shorter remaining warranty and less choice of spec. For many buyers, lightly used is the sweet spot between the depreciation cliff of new and the reliability risk of very old.

Financing: shorter is cheaper

Dealers negotiate on the monthly payment because it hides the two things that actually matter: the price and the interest. A lower payment almost always means a longer loan, and a longer loan means more total interest and more time spent “underwater” — owing more than the car is worth.

Three levers control the real cost: the price you negotiate, the APR (which includes most fees, unlike the headline rate), and the loan term. A bigger down payment and a shorter term both cut total interest sharply. Get pre-approved by your own bank before you walk in, so the dealer’s finance desk has to beat a real number rather than set it.

Lease, finance, or pay cash?

Paying cash avoids interest entirely and is cheapest if you can do it without draining your emergency fund. Financing costs interest but spreads the cost and leaves you owning an asset at the end. Leasing has the lowest monthly payment and no resale hassle, but you own nothing when it ends and face mileage limits and wear-and-tear charges along the way.

As a rough rule: buy (cash or finance) if you keep cars a long time and drive a lot; consider a lease only if you want a new car every few years and value low payments over ownership. Run both through the numbers over the same period — leasing always looks cheaper month-to-month, so the only fair comparison is total cost including what you’d own at the end.

The costs beyond the sticker

The purchase price is just the entry fee. Over the years you own a car, depreciation, insurance, fuel, maintenance, tyres, and registration add up to far more — often well over half the total cost of ownership comes from things other than the price you paid.

This is why a cheaper sticker isn’t always the cheaper car. A bargain that depreciates fast, drinks fuel, and costs a fortune to insure can easily cost more over five years than a pricier car that holds its value and sips fuel. Compare cars on total cost of ownership, not the number on the windscreen.

At the dealership

Negotiate the out-the-door price first, in full, before any talk of monthly payments or trade-ins — those are separate negotiations the dealer likes to blend together to confuse the maths. Keep them apart.

Be ready to decline the add-ons offered in the finance office: extended warranties, paint protection, gap insurance, and the like are high-margin and rarely worth their price at the markup offered. And never let a salesperson roll the negative equity from your old loan into the new one — it just moves a problem forward and grows it with interest.

Try the calculator

• Car Affordability Calculator — Find the car price you can actually afford with the 20/4/10 rule: 20% down, a 4-year loan, and total car costs under 10% of your income.
• Auto Loan Calculator — Work out a car loan’s monthly payment, total interest, and amount financed — including down payment, trade-in, sales tax, and dealer fees.
• Lease vs Buy Calculator — Compare the true cost of leasing versus financing the same car over the same period, including resale value, to see which is genuinely cheaper.
• Car Cost of Ownership Calculator — Add up what a car really costs — depreciation, fuel, insurance, maintenance, and registration — over the years you own it, per month and per mile.