Methodology

A car is not a one-time purchase — it is an asset you hold for a stretch of miles. Instead of asking "which car is cheapest over a fixed number of years?" we ask "which stretch of miles, on which car, is cheapest to own?" Every tool on this site is built around that question: model how the asset decays, price the friction of converting it back into cash, and find the exact mileage window with the lowest net cost per 10k miles.

We model vehicle depreciation using a shifted-exponential decay curve anchored to the vehicle's projected resale value. The curve is steepest in the early miles and flattens through mid-life, drifting toward a nominal scrap-value floor by end of life, reflecting observed market behavior for mass-market vehicles.

Maintenance costs are modeled as a non-linear ramp. The early miles (under warranty) are cheap, mid-life rises moderately, and the final stretch spikes as major components (transmission, engine, suspension) begin to fail. The ramp is calibrated to each vehicle's baseline maintenance figure and interpolated onto the mileage grid.

We slice ownership into twenty 10k-mile blocks and chart the cost of each additional block. Marginal depreciation starts steep and shrinks (the value is falling fastest when the car is newest), while marginal maintenance starts cheap and climbs (the car is aging). Their sum traces the classic U-curve: expensive early miles, a cheap trough in the middle, and brutally expensive end-of-life miles. The bottom of that U is the cheapest driving you will ever do.

Converting a car back into cash is never free. A dealer trade-in or instant cash offer is fast (~2 hours of paperwork) but surrenders roughly 4% of the vehicle's value as wholesale-to-retail spread. A private-party sale keeps most of that spread (~1.5% in listing, transfer, and inspection fees) but costs around 12 hours of your time — priced at your hourly value of time and amortized over the ownership window.

For every possible (buy, sell) mileage pair on the 10k-mile grid, we compute the net cost of owning that stretch: the buy outlay plus buy-side friction, minus the net sale proceeds after exit friction, plus every dollar of maintenance in between. Dividing by the window's mileage span gives a net cost per 10k miles, and the pair with the lowest value becomes the shaded Optimal Ownership Window on the chart. Comparing the best used window against the best new-car window yields the "New Car Premium" — exactly what insisting on a brand-new car costs you per 10k miles.