The usual advice — never spend more repairing a car than it is worth — compares your repair quote against nothing. The real alternative is buying another car, and that is not free: a new car in Australia loses roughly 15–20% of its value in the first year — about $7,200 on a $40,000 purchase — before stamp duty of $1,200–$1,700, transfer fees, and loan interest that can run past $7,000 over a five-year term. Counted properly, a $4,500 repair on a $9,000 car routinely beats replacing it by thousands of dollars over five years. Sources: published Australian depreciation estimates; state revenue office duty rates, verified July 2026.
Marcus drives a 2015 Mazda 3 with 160,000 km on it. The gearbox needs work: $4,500, says the mechanic. The car might fetch $9,000 repaired. His brother-in-law's verdict is instant — "half the car's value on one repair? Get rid of it." It is the most common piece of car advice in Australia, and for most people in Marcus's position it is wrong, because it compares the repair bill against zero instead of against what walking away actually costs.
The Rule of Thumb Compares Your Quote Against Nothing
"Never spend more than the car is worth" is an insurance write-off test. Insurers total a car when repairs approach its market value because they are cashing you out and selling the wreck — keeping the car was never on their table. Borrowed by owners, the test quietly assumes the alternative to repairing is free. It isn't. The alternative is acquiring another car, and acquisition is the most expensive thing in motoring.
RACV's car running costs survey — run for more than 50 years — finds that once loan repayments are counted, purchase price and finance can account for as much as 80% of the total cost of owning a car. Fuel, servicing, tyres, registration: all of it together is the smaller share. Keeping a car you already own skips the dominant cost entirely. That is the structural reason "just replace it" loses so often when the arithmetic is done honestly.
What Replacing Actually Costs — Before You Drive Anywhere
Here is the ledger for a $40,000 new-car replacement in NSW, financed over five years — the costs that exist only on the replace side:
| Cost | First year | Over 5 years |
|---|---|---|
| Depreciation (18% year one, then 12%/yr) | $7,200 | $20,330 |
| Stamp duty (NSW, 3%) | $1,200 | $1,200 |
| Transfer, plates, first rego | ~$800 | ~$800 |
| Loan interest (6.9% p.a., 5-year term) | ~$2,500 | ~$7,200 |
| Total before fuel | ~$11,700 | ~$29,600 |
Depreciation from published Australian estimates (15–20% year one, 10–15% ongoing — midpoints used). Duty from Revenue NSW rates, verified July 2026. Interest at an illustrative 6.9% secured rate — substitute your own quote. Trade-in value of the old car offsets part of this; the calculator nets it off.
Set Marcus's $4,500 quote against that ledger and the brother-in-law's confidence collapses. Even after netting off a $3,000 trade-in and budgeting a further $1,500 every year for future repairs on the old car, repairing and keeping comes out thousands ahead over five years — and the older car's own depreciation is counted in that. On the calculator's default scenario the gap is about $9,250 in favour of repairing, and the repair quote would need to pass roughly $13,750 — one and a half times the car's repaired value — before replacing became the cheaper path.
Enter the repair quote, what your car is worth, and what you would replace it with. The calculator counts depreciation, duty for all six states, and loan interest — then shows your break-even repair bill and what a second breakdown would do.
The Costs That Cancel Out — and the One That Doesn't
Most repair-or-replace comparisons pad both columns with registration, insurance, servicing and tyres. It looks thorough, and it buries the decision: you pay those whichever car is in the driveway. Insurance runs a little higher on a dearer car, servicing a little lower on a newer one — the two pull in opposite directions and rarely decide anything.
The exception is fuel. Swap a thirsty older petrol car for a hybrid or an EV and the running-cost gap is real money — at 13,000 km a year, moving from 9 L/100 km to 4.8 L/100 km saves roughly $900 a year at $1.80 a litre. Two honesty notes on that. First, pump prices swung more than 60 cents a litre during 2026 alone (ACCC weekly monitoring — a conflict-driven spike, an excise cut, then a partial restoration), so any fuel saving is an estimate built on a moving number. Second, the saving has to be large and sustained to outweigh the acquisition costs above — at typical driving distances, efficiency alone rarely rescues a replacement that loses on depreciation.
When Replacing Genuinely Wins
The arithmetic usually favours keeping — but not always, and cost is not the only test that matters.
- The repairs are a pattern, not an event. One $4,500 gearbox is an event. $2,000 three times a year, every year, is a car telling you something. Put the real recurring figure in the future-repairs budget and let the verdict flip if it flips.
- Safety has moved on from your car. Structural rust, degraded airbags, no electronic stability control on a car your kids drive. None of this appears in a cost ledger, and any of it can be decisive on its own. A cost calculator should not talk you out of a safety decision.
- You cannot afford the uncertainty. If being without the car costs you income, or an unplanned $4,000 bill would mean debt, paying more for predictability is a rational financial choice — not a failure of arithmetic. Certainty has a price, and sometimes it is worth paying.
- The efficiency gap is huge and you drive a lot. High annual kilometres in a very thirsty car, replaced by something dramatically cheaper to run, can shift several thousand dollars over a long ownership window. Run it with your real kilometres rather than assuming.
How to Run Your Own Numbers
Five inputs decide almost every case, and you already hold all of them — that is what makes this decision calculable where most car-finance questions are not.
- The quote — what the mechanic wants to make the car reliable again.
- Two values for your car — repaired, and as-is. The gap between them is what the repair buys you back.
- An honest future-repairs budget. Nobody can predict an old car's next failure — your service history is the best evidence there is. Whatever you set, test the answer against one more breakdown: if a single extra $3,000 bill flips the verdict, the decision is genuinely close and reliability should carry more weight than dollars.
- The replacement, priced fully. Purchase price plus duty — the car stamp duty calculator gives the exact figure for your state — plus on-roads, plus interest if financed. A used replacement changes the answer materially, because it skips the steep first-year depreciation.
- How long you'll keep whichever car wins. Three years favours keeping (the replacement's worst depreciation years dominate). Seven narrows the gap. Pick the horizon you actually believe.
The number to anchor on is the break-even repair bill — the quote at which replacing starts to win. For most people with a functioning car worth $8,000–$15,000, that break-even sits far above what the resale-value rule of thumb implies. Get the quote, know your break-even, and the decision mostly makes itself.