Repair or Replace Your Car? Settle It With Cost Per Mile

The transmission quote comes in at $3,100. The car is worth maybe $4,000 on a good day, has 168,000 miles, and the mechanic does the thing mechanics do where they tilt their head and say “it’s your call”. Everyone you ask offers the same folk rule: don’t put more into it than it’s worth.

That rule is wrong, and it costs people thousands of dollars a year.

The Folk Rule Compares the Wrong Two Things

“Never spend more on a repair than the car is worth” compares the repair bill to resale value. But resale value is only relevant if you’re selling. What the repair actually buys is miles of transport — and if you don’t buy them from the mechanic, you buy them from a dealer, at dealer prices, with a new depreciation curve attached.

The right question is a per-mile one:

What does keeping this car cost me per mile over the next N miles, versus what a replacement would cost me per mile over those same N miles?

Whichever is lower wins. Resale value doesn’t appear in that comparison at all except as a small offset if you sell the old car.

The Comparison, Worked

Below are my own illustrative assumptions, not survey figures — a $3,100 transmission on a 168,000-mile sedan, against a $16,000 used replacement, over a two-year, 24,000-mile horizon.

Option A — Keep and repair

Option B — Replace

The two options land within a percentage point of each other — which is the genuinely useful result, because it means the decision isn’t financial. It’s about risk tolerance, how much the reliability matters to you, and whether you can absorb $3,100 this month.

Notice what happened to the folk rule along the way. It said “never” — spend $3,100 on a $4,000 car? Absurd. The arithmetic said the two paths cost effectively the same, and that’s with a pessimistic $900/year repair allowance stacked on top of the transmission.

Change One Assumption and the Answer Moves

This is why running it yourself matters more than reading anyone’s conclusion:

• Keep the repaired car four years instead of two. The $3,100 spreads over 48,000 miles instead of 24,000, and keeping drops to roughly 47 cents/mile while replacing barely moves. Repair wins clearly.
• Assume $2,000/year in other repairs instead of $900. Keeping rises to about 65 cents/mile. Replace wins.
• Buy a $9,000 replacement instead of $16,000. Replacement depreciation and insurance both fall sharply. Replace wins comfortably.
• Drive 6,000 miles a year instead of 12,000. Every fixed cost — insurance, depreciation, registration — spreads over half the miles, so both options get more expensive per mile, and the cheaper-to-own old car wins by more.

The decision hinges almost entirely on two things you control: how long you’ll keep the repaired car, and what you’d actually replace it with. Neither is a fact about the transmission.

The Input That Decides It — and Nobody Has

Look back at Option A. The single most influential line is “other repairs: $1,800”. If that number is really $600, keeping is a landslide. If it’s really $3,500, replacing is.

Most people guess it, and they guess badly, because there’s no memory of the year’s repairs — only a vague sense that “it’s always something”. A repair log with dates and amounts converts that feeling into two hard facts: what the car has actually cost per year in repairs, and whether that figure is trending up.

Trend is the real replace signal. One expensive year is bad luck. Three consecutive years of rising repair spend, with failures migrating from wear items (brakes, belts, suspension) to major systems (engine, transmission, subframe, rust), is a car telling you where it’s going. You cannot see that pattern from memory. You can see it in ten seconds from a logged repair history sorted by date.

The Vehicle Expense & Maintenance Tracker from ReadySheetGo is built for exactly this decision. The Repair History tab logs every repair with date, odometer, parts and labour split and warranty status, the Maintenance Log carries scheduled service and next-due dates, and the Dashboard returns the cost-of-ownership summary and cost per mile that Option A above depends on — for up to four vehicles at once.

A Four-Step Version You Can Run Tonight

1. Set your horizon. How many more months, and how many more miles, would you keep the repaired car? Be honest — this number does most of the work.
2. Build Option A. The repair, plus your tracked annual repair figure times the years, plus maintenance, fuel, insurance and depreciation over that horizon. Divide by the miles.
3. Build Option B. Realistic purchase price, minus realistic resale at the end, plus interest, fuel, insurance and maintenance, minus what you’d get for the old car today. Divide by the same miles.
4. Run it twice. Once optimistic, once pessimistic. If the winner doesn’t change, act on it. If it flips, the decision genuinely is close — and close decisions should default to keeping the car, because that’s the option that doesn’t require signing anything.

For the wider picture on how each of these buckets is tracked, see the pillar: vehicle expense tracker spreadsheet. And if the repair pattern is what’s worrying you, the service side is here: car maintenance log spreadsheet.

Featured on ReadySheetGo

Vehicle Expense & Maintenance Tracker — The repair-or-replace decision needs a repair history and a cost per mile, and this gives you both. Nine tabs and 674+ auto-calculating formulas: Repair History with parts-versus-labour and warranty tracking, Maintenance Log with next-due reminders, Fuel Log with automatic MPG and cost per mile, Insurance and Loan/Lease tabs, depreciation against purchase price, and a Dashboard cost-of-ownership summary for up to 4 vehicles. Works in Excel and Google Sheets, no macros. Instant digital download — $12.99.