Total cost of ownership over a chosen period, for any number of candidate vehicles, measured against keeping the one you have. Petrol, diesel, electric and plug-in hybrid, cash or financed. Everything is editable, nothing is sent anywhere.
Opportunity cost is what your money would earn elsewhere instead of sitting in a vehicle — use an after-tax rate. The residual stress test shifts every candidate's end value by that percentage at once: leave it at zero for your central estimate, set it to −20 to see what a slump in used values would cost. It does not touch the vehicle you already own, whose value is usually near its floor already. The default trade-in only seeds newly added candidates — each one keeps its own value afterwards, since different dealers quote differently for the same car.
Each candidate is credited once with its own trade-in or sale value for your current vehicle — edit it per candidate, since different dealers quote differently. A second set of wheels and tyres counts as a one-off cost of switching, since you presumably already have both for the car you own — set it to zero if the vehicle comes with two sets. Replacement tyres are left out of every column on the grounds that they broadly cancel. Depreciation is purchase price minus your estimated value at the end of the period, and it is usually the largest line and the one most worth arguing about. The model assumes the vehicle you keep survives the whole comparison period; if it doesn't, you buy something sooner than planned, which this tool doesn't attempt to predict.