Build the ownership cost boundary

Use model-specific intervals

The owner's manual and service history are better inputs than a universal percentage of vehicle value.

Separate maintenance and repair

Scheduled work is expected; failures are uncertain. Keep the average repair allowance distinct from the cash reserve needed for a large event.

Illustrative reserve

Cost layer
$900 tires every 45,000 miles = 2.0¢/mile
Time or usage input
$600 scheduled service every 30,000 miles = 2.0¢/mile
Cost layer
$500 annual repair allowance at 12,000 miles = 4.2¢/mile
Time or usage input
Combined reserve: about 8.2¢/mile
Cost layer
Age and model-specific failures remain uncertain

The boundary should include only costs that change between the options being compared.

This decision belongs in The Real Cost of Car Ownership. Use Vehicle Cost per Mile: Use the Right Denominator to check the next assumption, then test the numbers in the Car Ownership Cost Calculator.

Adjust the ledger for the real decision

Account for time limits

Fluids, batteries, corrosion and rubber can age even when mileage is low.

Avoid double counting

If a warranty or service plan covers an item, include the plan cost and only the uncovered deductible or exclusions.

Build a complete cost boundary

A maintenance reserve converts irregular tires, service and wear items into a planning amount. Base timing on the vehicle, records and mileage rather than one universal cents-per-mile figure.

Records for the reserve schedule

  • Current mileage and completed maintenance history.
  • Manufacturer intervals plus known age-related work.
  • Local installed prices for tires, brakes, fluids and major service.
  • List scheduled work over the planned miles.
  • Convert tires and wear items to cost per mile.
  • Add time-based work for the holding period.
  • Create low and high repair scenarios.
  • Keep enough liquid cash for a full plausible failure.

Turn the cost boundary into a working estimate

Car Ownership Cost Calculator

Model the costs that continue after the purchase price leaves the showroom.

Costs that disappear from a monthly view

  • Using one annual average for every vehicle age.
  • Treating expected cost as the maximum cash need.
  • Counting covered work twice.
  • Ignoring time-based deterioration at low mileage.

Questions this decision usually raises

How much should the cash reserve hold?

It should reflect a plausible near-term repair and deductible, not only the probability-weighted annual average.

Does maintenance raise resale value?

It may preserve condition and evidence, but do not assume every dollar is recovered.

Should maintenance be per mile or per year?

Use both drivers where appropriate and combine them over the chosen period.

Sources for prices, timing and useful life

This page explains a calculation, not a personal recommendation. Contract rules and consumer protections vary by product and jurisdiction. Check the current agreement and official guidance before acting.